I don't think you can or should prevent your DH from trying again to have a relationship with his kids. That doesn't mean YOU have to have a relationship with them though.

It doesn't mean separate lives for you, not at all. More like you aren't taking part in something that he wants to do but you don't. Give him that space, but with boundaries. He must protect you and the marriage first and foremost, and that means no bashing you, no disrespect hurled in your direction even if you are not present, no financial commitments without discussion with you, no inviting them to your home. And your DH must respect and support the fact that you choose not to have a relationship with people who have treated you badly.

I'd be GLAD for my DH to see his kids and grands without me. He just doesn't. Haven't seen them in about two years. 

It happens. Some steps are able to blend others can't. I found out long ago when steps refused to come to our wedding or even before they refused to engage with me and my kids. So now DH has relationships with his daughters separately. Mostly by phone to be truthful. I don't lift a finger to do anything with his daughters or advise him to see them. One daughter we see a few times a year. She refuses to spend the holidays with us and my kids. Her loss. She hasn't been to our home in over a year. Accept things you didn't create and can't change. Their loss. 

Let your DH have his relationship with his kids away from you...away from your marital home. I've been in this mode for 5+ years. We did try a "reunification" a few months ago and while the one with son/wife/kids went okay, the one with the two daughters did NOT. So, I am back to "no contact" with DHs kids. He flies out once a year to see them in their area or if they come to our area, he visits them at relatives in the town they grew up in. 

We cannot blend. We never will. I've come to accept that and personally, I don't want anything to do with DHs ADULT children. They are people I would NEVER hang out with and the only reason they were in my life for the time they were, was because they were DHs kids. I've since learned I don't have to take toxic treatment from anyone, including SKs.

Search this site for disengagement techniques and find the ones that work for you. Best to you. This is not an easy journey.

Do not let him wallow in self delusion that his kids are not the undesirables that they are.

If he wants to wallow in that cess pool, that is on him.  Do not allow one Cent of marital resources to be spent on them or the GSkids they use to manipulate your DH.  Do not allow any fantasy reference to them to go unchecked without rubbing his nose in the facts.

Sadly, sometimes we have to facilitate painful lessons for those we love.

Theyve treated you BOTH with rudeness and disrespect. So you know the drill and your DH does not:

The drill of disengagement:

1 YOU do nothing for skids or sgrands. DH does ALL. If he wants to have his relationship with disrespectful manipulative harpies, he can go for it, without you or your involvement or money.

2. If he does this re-engagement with DMH (see above), then sit him down and discuss boundaries. Finances that are used on DMH are HIS only (except in some states his is yours). You can  definitely have together time with your bios and include him. Discuss all this calmly.

3. If hes acting out and lashing out, then you must call this out. I did and do. You can say something like "I know you are stressed out about the fact that your skids have been so horrible in their treatment of us, but please refrain from taking it out on me." and then walk away. Hes talking to you and angry? walk away immediately.

4. Focus your energies on your bios and enjoy this time with them and your grand(s). Try not to allow the emotions in there, because they can tear you up. Im currently working on this too, and its working. I am having less and less thoughts on things to do with skids, and am having more involvement in my own life. If Husband wants to hang with his kids, Im ok with it. If they want to abuse or use him, Im fine with it. He knows now that he cannot draw me in there with him anymore.

So there's really not much I can contribute.

Other than this which stood out: " I get that he wants to have a relationship with his kids, but I then don't because of the way he has been treated. Why put yourself forward for more abuse?!"

You've answered your own question. Your DH is willing to put up with abuse BECAUSE he wants a relationship with his kids.  That's the price he's willing to pay but that doesn't mean you have to be in the audience and watch it all happen. Disengage.

I've been disengaged for years and have also seen some of the pissy behavior that DH's like to exhibit. Mine is going to see his SD and her clan soon and I'm not going. He made a SLIGHT remark about me not going (because he wants me to be a buffer) but I made it very clear to him, again, that it is best he spends time with them alone.  And I say nothing more but walk away.

The last time we had an argument about it (years ago) I made it abundantly clear that I was not welcomed by SD, I never would be as she was now an adult woman with her own family/home and that's the end of the story. I would never insert myself into someone else's home when I knew I was not welcome, let alone subjecting myself to a toxic environment watching all the sh!tshow antics. I finished with the absolute, basic fact that he has such little time to spend with them it's best he enjoys every minute of it.  When we had this argument, he behaved like a five-year old who has had all his toys taken away and must sit in a corner - pouted and acted all butt-hurt for days afterward.  Tough. 

So now, I just use my bullet statement "You need to enjoy every minute with them." as shorthand for the above. He gets the message. 

but, he has to be on the lookout for any and all abuse from them.  I would hope, if this happens, he'll recognize it immediately and set boundaries and then leave whatever little gathering they're at. Also, I am hoping, for your sake, your DH really does want to be a grandpa to your bio grandbaby (congrats!!) and doesn't go out of his way to snub them because you don't want anything to do with HIS kids.  My DH has taken this stance with my son because I want nothing to do with his oldest son. I've confronted it and, if I'm being honest, it does cause me to feel resentful. Honestly, if I knew 2nd marriages and stepkids were going to be this much work, I would NEVER have re-married. Good luck and most of all, enjoy the beautiful little baby you'll soon be holding

Regardless of who those toxic people may be.

That so many SOs to SPs willfully expose themselves to toxicity that they are the breeders of, then expect an SP to dive into that toxic shallow and polluted gene pool with them is disappointing at best.

I would tell him that the money he gives them comes out of his "fun" budget as does the time he spends with them. He'd better not miss a single date night! 

Very very similar story here. My biracial adult step kids accused me of being a racist classist after several incidents where the observed behavior that was "too white." One was when they demanded to buy DH's house for less than half of market value.
After 2 years and over $16,000 of marriage and individual counseling I still wake up thinking about my 2 estranged adult stepsons. It's been a nightmare. When my DH goes goes to events (birthday party of grandson, nature walks, etc) I ALWAYS leave the house. Sometimes I am gone for one day, but up to a month. I am lucky to have places I can visit on short notice. It feels like DH is having an affair. I dwell on this hourly, every day. These sons were adults when I met DH, who like me had been divorced for a decade. It's awful. I don't know if the marriage is sustainable, as I have not been able to rise above it. Yet I love DH and hate to leave. I treat my absences as self care, and find friends and activities on my own. This has created an enormous gulf between me and DH. I feel your pain. PS - I do daily work, reading and study in the white privilege/systemic racism subject, and am on a path of lifelong learning. (I am white).

It's difficult. We had a good relationship with SKIDs for some time, my BioKids got on with them too. But it's been a few years now and became so toxic. BM manipulated them and whilst I can feel sympathy I am now working on putting up really high walls because I'm over trying to be kind, and thoughtful, and getting nothing but toxic behaviour in return. 
When my parents divorced there was a decade of nastiness, bitterness, pettiness, and parental alienation. I know people tell themselves they're doing the right thing even if it's toxic. My siblings and I were caught in the crossfire. I will not be part of that cycle again. 
But it's important for my DH to keep his relationships with his kids and grandkids. He sees the toxic behaviour for what it is, and I think that's hard for him. 
It's early days for me working out what the boundaries are going to be. But I'm no longer doing things for them, like buying presents for DH to give them. And DH will see them when it is not eating into our time together.  When he sees them (if it's outside work hours) I will do something nice for myself, so I don't associate it as much with negative feelings. 
He will only attend events without me if it's something I really feel he should, like weddings. I will not be the bad guy who says he can't do a wedding or a graduation. But things like birthdays, he can catch up with them the day before or day after, he won't be attending as part of a group that excludes me. 
Sadly for them, he sees much more of my Bio kids and grandbaby. That is the repercussion of the cruelty and nastiness they have chosen to send our way. There was a time where we saw them as much as we see mine. 

Work out your boundaries, enjoy that new grandbaby.