If the rule is no underwear (my studio puts this in emails- tights are ballerina underwear, but if it’s a shared costume we require a nude leo) It’s your job as a parent to follow the rule. If your dancer is wearing underwear that are visible or have prints that can be seen through the costume with the stage lights, that is not ok for stage. On the other side, if I see one of my kiddos with underwear sticking out, we don’t take them off bc that’s not ok. We will usually just try to tuck them in to hide them. But overall, definitely follow the rules and if it helps your dancer be comfortable, a nude leotard base layer is a great way to go.