I think it would be best to speak to the 10 year olds Parents as this child will do it again to others perhaps he has learned the behavior, so you could try to get some help to make sure he doesnt go on to do it again. Do not let them be together the cousin really would be best staying away from your home .,more supervision when children play together, too many times parents ,to get some peace leave children of differant age groups alone,.Be aware of what they are doing when you have them over .to your house .