If my son was injured due to horseplay, are the other kid's parents liable?

If the horseplay was mutual--that is, it wasn't the other boy shoving or pushing your child when he was trying to get away, for example--then it is very unlikely you could recover compensation. For there to be compensation, there must be fault; fault in a case like this is judged by a standard of negligence, or unreasonable carelessness; what is "unreasonable" is judged by the standard of the age of the participants. Simply put, boys horse around; when they do, one may accidentally get hurt. If the other boy did not deliberately push your son's face into a window sill with an intention to cause injury, it is unlikely that it will be found that he was negligent, or unreasonably careless, by indulging in horseplay with your son.