Bruise On Child

I am going anon and trying to post vaguely in case I need to file a report. A child that plays with my child came over with a big bruise on their back/bottom today. The child was squatting down playing with chalk & had a little plummers crack. I dont know how long it is because I'm not going to look at the child's bottom intentionally but it looked like a belt mark slightly up the back. I do not spank. Spanking is one of those things that makes me cringe. My child is extremely well adjusted and kind. This child isn't unkind but he's definitely lacking empathy and it's caused problems within play. I knew the child's parents spank and for that reason I do not involve the parents with misbehavior. Now I am concerned this is legal abuse. However, since I am bias I want to get feedback. The bruise is a black ish blue.