I would keep him away from the other children, even if this means you have to watch him every second and not be able to relax as much. I would also ask other trusted friends in the church to keep an eye out for unacceptable behavior. If you can get other parents on board with correcting the naughty kids and protecting your child it will make the environment safer.

And don't feel afraid to discipline the children hurting your child. You have a right to speak up and say "Do not hit, hitting hurts and it's not OK", etc. If their parents give you any flack about it let them know that your child's right to be safe trumps their kids right to misbehave.