I was extremely close to my grandmother - and I also grew up with my father warring with his brothers over one thing or another. So I'm writing this from the perspective of someone who has been where your kids/their cousins are.
When my grandmother died, my younger cousin forbade me to attend the funeral. He spent the first 13 years of his life (and he was 17 when she died) actually not knowing our grandmother at all - because his father had estranged himself from the entire family. I was 23, and had a small child, so whilst extremely upset that he'd threatened to "kill" me and my daughter if we went to the funeral, I didn't go. I figured that guilt was the motivator and possibly jealousy as he was 3rd oldest and both 1st (me) and 2nd (another cousin he threatened - except she was at the funeral at the time) were very close to our grandmother. I didn't want to have a scene caused, and - well, it's not like my grandmother was actually there is it?! My child and I spent the day at the local farm, believe it or not, actually doing what I know my grandmother would have wanted - having fun together, rather than moping around and arguing over who the next "matriarch" is going to be.
You cannot get involved here. For your kids' sake. They already have a violent ex-offender for a father... they don't have that side of their family anymore! Key word being "their". You get involved when their father's already says that he doesn't even want his kids to attend - and it really isn't going to end well. For. Your. Kids.
As someone else said - either your teenagers already have some semblance of a relationship with the kids of the woman whom their dad assaulted so badly that he was locked up for it... or they don't. I'm guessing that they don't, or they pretend they do for fear of upsetting you. It doesn't matter in the grand scheme of things. I'm not close to those cousins because our fathers aren't close. It doesn't impact upon my life one little bit (except to be grateful that I have no relationship with the violent thug who's the 3rd oldest - but oldest grandson, which my grandmother found laughable even when she was alive!). I have other cousins, a whole heap of friends, and a father who bit his tongue and let me be the catalyst of his nephew's guilt, jealousy and narc rage... I will probably never forgive my parents for putting me first, over a funeral that ultimately means nothing.
There are other ways of paying respects.
One would be winding your neck in and trying not to recreate the awful choice your EX-MIL made when her son assaulted her daughter so badly that he got locked up for it.
for your kids. But there are better ways to say goodbye, and/or maintain relationships going forwards (although, frankly, I think that ship sailed when your ex hurt the mother of your kids' cousins).
PS: You're only the matriarch of your children... and only then, if THEY want you to be.