the moment I read that you are afraid he may harm your children to get back at you had my alarms ringing because in most cases when children are hurt, the mother had usually voiced her concerns yet no one had taken them seriously. As his ex partner you know him better than most and if you seriously believe he could harm your children as an act of vengeance (the usual reason in most cases) then you MUST start making lots of noise, lots and lots until you are heard.
If it is in dispute are you able to have the courts test him?