How old are they? sounds like they are a little older. If this "rough" boy is anything over 2 years old...he should know better than to kick a person in the belly. Not to mention, knowing that he is a "rough player" i would also be concerned over his "intent" although you didn't see this happen, so of course you can't jump to conclusions, I am almost willing to bet this boy knew you were out of the room and knew exactly what he was doing. I have NO tolerance for "mean or rough" play...I have had parents tell me "oh that's just how boys play"...BULL! thats how MEAN boys play. Play should never involve "hurting" someone else, jumping on each other and kicking, is not play...it's voilence. Watch that boy and watch the way your son reacts to him, he very weel could be a little to rough, or just plain mean. Some kids like to hurt others, they like the reaction or power. I have a 4 year old who likes to intimidate other kids when "he thinks" i'm not watching, he will walk real close to them and stick out his chest and kind of bump into them, with a "nasty" look on his face. Little Creap! I ask him how he'd like someone to do that to him, (he is deep down quite tender) he of course says he wouldn't like it! So why do that to someone else? Hate to be so harsh, but kids by nature can behave quite "nasty". I say: watch him...if he portrays more angry outbursts or anyone else gets "hurt" while in play with him...let him go. Thats not anything you want in your house or done to YOUR child...we do this job to make life better for our kids. right? I don't like bullies, sounds like that might be what you have!