I think you handled it the right way.
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A follow up would maybe do good, just to kind of push the police a little to make sure this guy and the kids as well get what they deserve. If he would do this in public no telling what he is doing at home.Him lyiing about it might have been the spark for me and as others have said, that would NOT be the desired course of action for many reasons. Well done. Call the officer that took your statement and see if you can get a follow up if it will help you put it behind you.
Was there obvious marks on the child, if so "in defense of a third party"
You stepped in and did the right thing. Good job. Most people would have kept walking.