I think I am alone here with you but NTA.

My parents have cut off family left and right off and on my whole life. And tbh, for most of them, when we DID see them, I immediately understood why they'd been cut out before.

I am so sorry for so many things.

1. Your children not trusting your judgment on family matters
2. Your partner:
   1. Not handling his mother
   2. Still seeing his family after they treated you so poorly
      1. still seeing them so GODDAMN OFTEN, wtf??
   3. Being okay with a mother who did not give a flying FUCK about HIS family, only her own offspring (which is weird af)
   4. Not defending the choice to your children
3. Your children assuming that should they have a relationship with their grandma, that she would just give them things.
4. Your MIL hoarding her children like a mother dragon