Then you can notify Facebook that xyz account was opened by your child who is younger than 13 and you would like them to close the account.
However, once again, I think you're getting worked up over something that's outside of your control - and making an issue of it only makes you look petty. If Dad is posting things on FB that the kids can only see at his house, why is that any different than him saying something about you when the kids are at his house. Granted, he shouldn't be doing that, either, but some battles aren't worth fighting.
Now, your other thread implies that illegal or dangerous activities might be going on. If that's the case, the response might be different but it seems that this thread is about whether you should stop Dad from saying bad things about you on FB.