Original families hire babysitters or dump them on grandparents.  He's just being a stick in the mud.  Using the skids as an excuse. There's no reason he can't leave them alone for a few hours at their ages.  When does he think they will be old enough?  (Eyeroll).  

It is not at all unreasonable to ask your fiance to leave his kids at home alone so he can accompany you to a social event.  In fact, it is unreasonable of HIM to refuse to go with you.  He's being a lousy partner - accompanying one's partner to social occasions that are important to them is part of what a good partner does.

11 and 13 year olds are old enough to be left alone for a few hours, and if he doesn't want to leave them alone, he can find someone to stay with them.  

Is this the way you want to live - having to go to parties and the like alone when you have a partner who is available to go with you?  That would not be acceptable to me.  DH and I try to make our plans to go out when we don't have the skids, but if something comes up when we do, we leave them with a sitter or a relative and go out together.  I wouldn't tolerate being left to go alone.

It doesn't sound as if he thinks they're too young to be left alone, rather that he doesn't want to spend a minute away from them, right? In that case, the kids are going to this party whether they want to or not, whether they'll be bored or not. Why? Because if they don't go, he won't go, and you shouldn't have a pair of children deciding on whether you can go out or not. So make them go and if he protests, remind him that he should consider your wants as well as theirs and that this isn't an every weekend event, it's a special occasion. Either he treats you like a partner or he stays joined at the hip with his kids and suffer a drop in your esteem.

That's what babysitters are for - adult couples to attend social events together. I'd point blank tell him, if you only have a part-time partner perhaps you ought to hire one to complement his off hours or hire a full-time one instead. 

Is it possible the reason he wants to stay home with them is because he is a part-time father and prefers to spend that time with them?

Yes. You should be annoyed - this is not a big ask. This is actually a very normal thing for couples to do for one another. You're in the right and he's being unreasonable and emotional to think the kids can't handle a couple hours on the ipad while you attend a function. Perspective? I babysat at 12! Literally, babysat two kids on my own at the age of 12- so not only in charge of myself but of 2 other living beings. 

And no, it definately is NOT unreasonable for you and DH to have time alone or with friends whole an 11yo and a 13yo are home alone for a few hours.

DH had the SDs 50/50 early on, before the PAS. He would never leave them so we could go to dinner alone. Refused to hire a sitter or anything because, I think, of guilt and paranoia that BM would take them back again. ALSO the SDs would literally have fits if we tried to do things alone. Bloody screaming and yelling. So I simply started doing things alone, like going out to dinner by myself once a week. When SDs we're around 10)12 he was still reluctant to leave them. OSD had her own phone, they were completely safe. It took me baby steps to get him out like normal people....intact families do this, we need to as well.

He backslid a bit after OSD PASd out. Didn't want to leave YSD12/13 alone. Not having it. YSD hates going out to eat so she's not coming with us, then she can make her own dinner and we go out. DH ended up loving time out. 

When we were overseas, our first night there, the SDs (15 & 13) were knocked out by jetlag. Fast asleep quick. We were not. There was a pub literally across the street and heck yes, we can get a drink and some food and be close. DH, once we were there, freaked. Wouldn't order food, wouldn't finish his beer because he was worried about SDs. "So what about me? What I need doesn't matter here does it? Isn't this also our vacation?" Nope, we rushed back to still sleeping SDs, and watched a TV show. After that I didn't help with anything on that trip (I wasn't helping with SDs then anyway but this was not helping him either).

As an infrequent event, the partners need to do this type of thing together whether or not the skids are on dad's time.

Dad and the kids should also have their time together with dad's mate participating as the usual during visitation time.

The preponderance of dump the Skids and go out situations tend to be primarily a CP characteristic by my calibrated eye ball.

As presented in the OP.  It makes sense to me for all of them to go to the event as kids are not excluded.  Whether the kids want to go or not.

I guess I would like to know how he spent that quality time with the kids?  Was he vegged  out in front of the tv while they were on their phones/ video games/ tablets?   Unless he was actively engaged with the skids while this event was going on , I'm on OP 's side that 2 hours wasn't going to kill anyone if he went. These child centric attitudes only enable the coddling and if you've been around her enough you know that's a major complaint.  Let's hope we get an update 

It may also depend on whether the kids at that age are mature enough to be left alone.. 13 and 11 is on the edge where you could have some concerns if they aren't particularly capable kids.

My vote would be to go.. even if it meant separate cars so your DH could eject his kids out of there if they started to act up.  maybe he could go for the first part then gather the kids and leave.. but if there are other kids their age there? maybe they will actually want to stay.

This is onHIM for telling his kids.. you will go.. you will behave appropriately and not embarass us.. not a request.. it's an order.. even if he lets them just sit to the side and play on their tablets while you two enjoy the outing.

Seems perfectly reasonable for you and your partner to go out for a couple of hours. Invite the kids....if they say no, then your H can get a babysitter in for two gd hours. What's wrong with that?? I get he looks forward to seeing them...but you are asking him to go out for a one off event, as your partner. 3 hours max.

of the other time requirements in a family. It is merely a time element that has to fit in to the whole.

 Intact initial families have Partner/Parental time demands.  Couple time, family time, homework time, play time, some 1:1 time between a kid and a parent, and broader Parent(s) time with all the kids, etc....  Blended families are no unicorn that changes these requirements though there is certainly added complexity.

Visitation is not time that the visiting Skid has full and unfettered 1:1 time with the NCP.  The NCP has a spouse, a broader family, and any number of other life elements that require time investment.  For that reason, a visiting kid has to integrate into the routine of the rest of the household/family with some limited 1:1 parent/BK time as close to how that looks in an intact initial family as possible scaled to how much time the Skid is actual in the NCP home.  

The rush to coddle the poor COD visiting Skid causes so much angst and drives a huge part of why second marriages fail at a higher rate than first marriages.  Is a large part of SParent unhappiness, and is also a huge part of the manipulation that far too many Skids attempt when on visitation and sadly when they return to the CP home.

IMHO of course.

Is it possible that he's scared that if he spends any time at all away from them that BM might use that to change their CO and go after more CS? 

No it's not unreasonable. Couples hire sitters or leave their kids with trusted family members so they can spend time together as a couple. How is your relationship otherwise? Do you feel he enjoys spending time with you or does he enjoy more what you can do for him and his kids?