I would talk to your ex´s wife and see what she says. You should all sit down and talk. When all facing each other, the truth will come out, if she hit your kid or not. If she really did hit your kid hard, you have a material to press charges. I wouldn´t allow anyone, literally anyone to not even scream at my child. And your ex´s behavior is also questionable. Personally I would seek the truth in all this and if it is true that she hit your child, I would seek full custody and forbid your ex from seeing the kid. If he allows the hits, then he is not deserved to be called the father.