You should speak with a personal injury attorney. Whether or not you can successfully sue and recover depends upon whether the injury was caused either deliberately (i.e. he tried to hurt her) or unreasonably carelessly (which is negligently). Negligence is judged in terms of the situation and also the age of the people--if the cousin was 14, he may well have been negligent, if she said to stop; but if he was 4, then he might not have been; the standard of behavior and care for a 4-year-old is appropriately lower. Experienced personal injury counsel can listen to all the facts and advise you as to the existence, strength, and value of any lawsuit or claim.