My ex says that my children are my problem and laughs when they misbehave?

You are letting him take control you go lawyer write him letter saying when he sees children and for how long for he does not turn up at your house ok and threaten him court order. You are not in his life you do not owe him anything you change your locks and he does not enter house when collecting children you hand them over at door end of he sees them and if he messing up your deny him access and if he takes you to court you have the copy of letter and show you are reasonable and he will look like the git not you