Do you know if the police were called or a report was filed ?
Personally I would stop visitation NOW. They are old enough for a judge to listen to and if the act was more than just a slap, then you should be able to go to court and change the visitation to be supervised.
As for legal action, you have two choices. Wait for him to initiate legal procedings or start them yourself. Call some local attornies and discuss it with them.. you can generally get a free 30 minute consultation with most attornies.
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SMILE - Start Making It Livable for Everyone
Divorce is a process over which children have no control. Children should not be its victims.
When parents are under stress, it is harder to be in touch with their children's pain and anguish.
It takes time, effort, and planning on the part of the parents to be able to provide for the children's needs.
In the crisis of divorce, parents may put their children on hold while they attend to adult problems first.
Sometimes separating/divorced parents find that their roles and expectations are undefined and cloudy.
If handled properly, divorce need not be devastating for children.