Grandparent rights

I don't know if one isolated event of the grandparents arguably exercising poor judgment in letting your son play with or light fireworks, is enough that the court would change their visitation schedule or omit it. What type of fireworks were involved? How would you prove it in court? If it is based solely on what your 7 year old told you following the visit, that is objectionable on  evidentiary hearsay grounds in court; you cannot testify in court what your son told you out of court, unless there is an exception to the hearsay rule. I would discuss your concerns politely with the grandparents so you can get their side of the story and you let them know you disapprove of your son at such a young age , handling fireworks. If they continue to disregard your wishes and expose your son to arguably other safety  hazards, it may be time to take this back to court to have your concerns addressed before the court.