While I would be horrified if my husband hit my kids, I would think twice before making a police report. First of all, the police will notify DCFS and you do not want DCFS in your life unless you absolutely have to to keep your kids safe.
I would talk to your ex and lay some ground rules about discipline and physical contact, his involvement with the kids and sports and if on reflection, he thinks what he did was OK. I might threaten to call the police or DCFS if this happens again, but unless this was a very serious incident or you have other safety concerns, I wouldn't. I might tell him that he needs to go to counseling or anger management classes if he wants to continue to see the kids. I might also want to meet with a counselor with him so that you can work out your discipline strategy together and the role that sports is going to play in your son's life. What is up with disciplining kids around sports? It's supposed to be fun!
I would not ask my kids' opinions of what you should do. That's putting way too much responsibility on them. I think talking to them about what happened--how they're feeling about it, are they scared of him, do they still want to see him, etc.
All of this depends on the seriousness of the hit. Was your son hurt or bruised? Even if he wasn't, your ex's behavior is not acceptable.
This is hard situation to be in--let us know what you decide to do.