It seems really strange to tear up your legs on mill circles, though it is certainly common when a gymnast is learning front hip circles and back hip circles. That said, I imagine if she is rubbing her legs against the bar in a mill circle, it is because she is not pushing away from the bar with her legs in a split position, but rather, squeezing her legs around the bar or hooking her knee. You want her to imagine pushing up and away with her legs open like a split (which I am sure her coaches are telling her).
If she is really bruises/scrapped up, you might want to ask her coaches about letting her wear shorts for bars only for a week to give her skin some time to heal. A lot of young gymnast go through phases of tearing their hips or legs (and of course, hands) up on bars, and they all turn out okay in the long run.
If the shorts are an absolute no go- a little chalk on the "ouchy" areas can help reduce friction.