I don't know much about this subject -- fortunately. I am sorry that you are having this problem.
It's my understanding that the best place to get a PI referral is from your divorce lawyer. PIs market to divorce lawyers in hope of getting referrals.
Not everybody needs a lawyer to get a divorce, especially if you have an amicable, cooperative divorce. But what you describe doesn't sound too amicable. Using the kids as a bargaining chip is a a common, but despicable, practice. If you don't have a lawyer, time to get one. If you do, ask for a PI referral.
ETA: I see now that you refer to her as your EX, so that means you are already divorced. Sorry I did not pick up on that. Still, if you believe she is violating the terms of the divorce agreement, the person to contact is the lawyer who represented you in getting the agreement.