Although the court has ordered my ex-wife to allow me access to my children, she violates the order.

Answer

Islamically you are the Guardian of your children. Your ex-wife is the custodian. It is haraam for her to deny you visitation and access without valid reason. As long as you are not a danger to the children nor a threat to their Islamic morals, she has no right to deny you free access at a neutral venue. If she is in violation as you have mentioned, you may take legal action against her.