and how would you know the person he produces as his AA sponsor is really his AA sponsor? it's not like they dispense badges or anything.
no one is trying to downplay the concern you have regarding your ex and the safety of your children. it's just that trying to involve someone else from AA is not going to do a damn thing to truly protect the kids.
you have SoberLink which is designed to assure a sober period.
your own children should be empowered to contact you immediately if things are not right or safe when they are in his care. it's advisable to include the school counselors so the children have another source of safety.