I have no idea what kind of person Gene Hackman was, or his kids are. I suspect the kids weren't happy that their Dad married someone who became first in line for any inheritance. A lot of drama, and disapproval from the kids about Gene being happy with his wife, and Gene cut his entitled kids out of the inheritance. Then the kids cut off any contact with their Dad, because they felt slighted after behaving like petty children.
Or maybe Gene was abusive to his kids, and they couldn't wait to get away from him.
Or maybe neither is what happened.
It's all speculation.
But if I had to bet, I'd bet on the first scenario, as I've personally seen too many examples of that to recount them all.
I know one thing for sure: I won't be spending any more time contemplating what happens in this sad situation.