The law here used to be under 50cc: 49cc and under was a motorized bicycle. No license required for that. It may have changed.
Not sure about there. The police need to know the law, though, and they may have been right, but to cite it, pull it up chapter and worse, would help settle disputes like this. All too often, they seem to just make something up, and that weakens their position and makes them look like they should be wearing jackboots instead of shiny black cop shoes. No need to get physical, a definite need to follow the 4th, and threats to the kid come off bad.
Whatever the problem, that did not come off well. My question is one of what was going on with the (assumedly) father?