Is there a divorce agreement or CO? How old are these skids?

First off, it is not YOUR job to fix anything. You keep saying what YOU are doing about it but what is your SO doing? Is he allowing her to keep the kids from him? Is there a CO that he can enforce? If not, he needs to get one. Sounds like your SO has sat on his butt and done nothing so far.
The kids are being PAS'd and your SO has done nothing to fight her. She clearly is getting her way which is why it is continuing.
You need to disengage. You have NO reason to call the kids and ask why they aren't coming. EVER. That is up to your SO.

Step away from the drama and let your SO handle it or not handle it as he sees fit.

And reconsider whether this is something you want to deal with for the rest of your life. Do you want to consider bringing children into this nightmare? I would read many of the posts on this site and really take a good look at whether this is how you want to spend the rest of your life.

I have been in my SD's lives for 4 years. She is now almost 11. You'd think she would be old enough to think for herself, but it doesn't work that way. My SD lives with me and DH full-time. Her BM has let her down so many times and recently lost all visitation with her. Even still, my SD believes anything that BM tells her and our BM uses this to her advantage. The only thing I can tell you is: You will NEVER win if you contradict her. They will always believe their mom. And, it sounds like she will stop at nothing to hurt your SO. I would do what the others have said, take her to court and get a C/O. Then follow it to the letter. If she doesn't comply, call the cops and have them go get the kids. At least this way, they can see you. But, even so, don't expect them to stop believing their mother. My SD hasn't lived with her mom in 2 years, but she still believes everything BM says.

This is very similar to what I recently posted and am hoping for a good response. How do you counteract pathogenic parenting? What can her and her DH do as a team to counteract the BM's poison? Get a CO or use the existing one to enforce EOW visitation. Ideally there is no CO and you get a CO and hopefully get more than EOW. Hopefully you get full custody so they barely see their mother to be poisoned by her. But lets say all you get is EOW. Fine. Enforce it obsessively no matter how many times you need to call the police or take the mother court for contempt. Okay fine, now the angry spiteful PAS'ed kids come EOW kicking and screaming while their mother continues to fill their heads with garbage.
What should the OP do about the fact that BM has turned them against the OP and her DH? How do you fix that?
Taking the high road is useless.

Confront the kids with evidence and truth is one somewhat reasonable solution I have heard.

If I had to wager a guess, I can say that if you trash talk the mother in return, it may backfire on you because she has them the majority of the time, so they will always believe her before they believe you or even your DH. She has way more influence than he does because of the custody arrangement, even though they are both their parents.

When my parents split my father had custody of me. My mother did trash talk him somewhat but I always sided with dad, mostly because I lived with dad 26 days out of the month and saw him behaving reasonably 26 days of the month. So when mom who I only saw 4 days of the month said otherwise, I assumed I knew dad better than mom did. My father never complained about my mother, so I cannot say how it would have effected me if he had. My stepmother did complain about my mother, but I realized that my mother and stepmother had limited interaction, so my stepmother didn't know her as well as me and must have been mistaken. Plus I knew she was the new woman so she just naturally came off as jealous and unreasonable so I certainly would always listen to mom first. Now I lived with her, so I just tuned her out. If only saw her EOW I might have lashed out at her for talking badly about my mom, I am not sure, so be careful.

I think in order to win this you need to be manipulative and smart. It will also be very helpful if your DH can take the lead on this, because his word is worth more than yours to the children.
You will need to teach the kids what they need to know that will eventually allow them to see through her-- but you need to do it without severely trash talking her yourself, because that will only make YOU look bad to them.
You need to do these things gradually over a period of weeks and months, and not all in one sitting.
Start by talking to the kids about divorce and how good parents are supposed to behave afterwards without specifically pointing out what their mother is doing.
Talk to the kids about relationships and how people can somehow behave unreasonably afterwards because they are hurt and angry.
Talk about how DH is much happier with you because him and BM just weren't very good together and are both better off apart.
Talk about love and families and how reasonable people should behave.
Talk about how damaging it can be to children when one parent says bad things about the other.
Talk about children of divorce being torn apart by loyalty conflicts when their parent cannot get along.
Talk about the importance of honesty and of always having a way to back up what you saying.
Even talk about how parents who really love their children will behave, and how some parents only love themselves and are willing to hurt their children to make themselves feel better.
If getting an audience is difficult, then talk to your DH about these things. Frequently. When the kids are in earshot, such as whenever your in the car with the radio turned off without making it clear that you're talking to them or about them or their mother. Get the message across. Comment on peoples behavior when watching movies if you have to. Make sure the kids are exposed to this stuff over and over again until they adopt the beliefs themselves even if having a conversation with the children directly would be a fools errand.
Once you appear to have made inroads with these things, you need to point out or imply that BM is doing exactly what people are not supposed to do without getting carried away and complaining about her insanity until the children tune you out. You need to bring it up in ways that allows them to make the connection themselves, hopefully before it comes out of your mouth.
And if you have any pull or ability to do so, put the kids in therapy.

If all of this fails, meaning if you've been doing these things for 6 months to a year and the nightmare continues on and on, you need to start talking to DH about allowing his kids to not come anymore if they don't want to in order preserve everyone's peace. The mother will damage them less if she feels that she has won, and the damaged children will hurt you and your relationship less if they're not around. Literally everyone benefits. You're not asking your DH to abandon his children, you are asking DH to do what is best for his children in light of what the mother is doing and give his children what they want to preserve everyone's peace and happiness. He needs to know when he's lost. Children who have rejected one parent because the other is a nutjob often come around when they are older.

This is just my best guess. To be honest with you, I don't actually know what to do. I have a similar post in the coparenting forum asking for advice on how to counteract this kind of thing. My issues is still the in the very early stages but my BM is acting just like yours or worse, so I am afraid of ending up with the same problem as you a few years down the line. I am hoping for answers to my post from someone this has happened to and who has won in the end, so I can hear about what they did that worked.