I am curious. Did you do their housework for them?
I am very sorry for your situation; I have two friends with similar destructive daughters-in -law who use their children as bargaining chips; both husbands are too weak to stand up to them and do not realise these women are also destroying their careers. They, obviously, do not work.
The latest request for child care sounds a very deliberate attempt to destroy your relationship by demanding something you cannot provide. Your son, sorry to say, is a fool , but try to maintain contact, however slight. The grandchildren, in both cases above, have resumed contact with their grandparents as they have grown older, built on happy memories,and also on the parents' need for financial support; one salary is not enough.Don't disinherit your grandchildren, but quietly rewrite your wills with regard to their parents. One DiL told her children that Granny had been stealing her money, which is why they no longer saw her.