Where do i find answers about non custodial parent visitation that is not in the divorce decree?

My daughter goes to a gymnastics cl a couple of times a week after school. Her dad (my ex) and his wife (well usually just the wife) come and watch her in cl. But they don't show up until about 5-10 minutes before cl is over. They just come to say hi to her. His weekday night with her is on Tuesday and these cles are not on Tuesday. Is he even allowed to come? I want to tell them to quit coming, but i didn't know for sure if i could even do that.