No contact with children

How can one initiate the no contact when there are children involved? My "kids" are 14 and 17 (the other two are considered adults at 19 and 24). We supposedly have "shared custody" which means they live at his house, he gives them expensive toys (cars, ipads, trips to the beach, unlimited cash), no curfew and no chores. He "forces them" to come see me, which means leaving unlimited tv and video games to come to my house where I enforce video limits and I don't have tv.

The involvement with him makes me sick. The kids lie to protect him (don't tell your mother, it's my money and I can do with it as I want), he encourages them to lie (if you tell your mother, she'll be really angry!), he undermines everything in our family life (pick up this room, you know your mother is a bitch and she'll punish you if you don't.)

If I don't contact him, the court will consider my lack of involvement abandonment. If I do contact him, I will lose what is left of my mind.

I know, eventually, they will see through his bullshit, but until then, I have been a stay at home, attachment style parent, the only parent these kids have known for 23 years, and all of a sudden Mr. My Parents Died and Left Me a Lot of Money is in the picture.

Are there any suggestions for maintaining sanity with this person? It is agonizing to lose my children to him, he wanted nothing to do with them until it became a game for him to see how much he could hurt me.