i have no idea what your life is like, some men are terrible to their partners but would never hurt their kids, I’ve no clue what your dynamics are.
Going against the grain, in the normal run of things I think it’s absolutely NOT unreasonable to trust a father to look after his own children’s money - I’d say that’s fairly normal.
What you’ve learnt is, he’ll treat them as bad as you, I don’t think you can go to the police or small claims, because as executor you released the funds to the wrong name and could get yourself in trouble too, I think your stuck with it, unfortunately.
If he does not/will not give it back, if your kids are young enough, try and put aside a small amount each week replace it, could be a tiny amount to do this if they’re under 10.
And heed the lesson, you now know he cannot be trusted to put his own children first, let alone anyone else. This is the time to find a way to leave.