Just do it. Plan to take some time off away from the festival to entertain your child, if it becomes necessary. For a seven year old, it may very well be necessary. Not because of a lack of things to entertain them, but because they need to mellow out, away from the hustle and bustle, and chill with their parent(s). That seemed to be the one hard and fast rule for us with small kids. As long as they are that age, you shouldn't have it all your way, or no way.

Outside of that, I too favor the "let them run wild" philosophy. There aren't too many circumstances as safe as a festival grounds, with as many kids, with as many wonderful new things to experience. But then, both of us parents were raised in large Catholic families that ran wild without adult supervision, with huge gangs of children, all summer long. And loved it.

If you have concerns for your particular seven year old running wild on their own, take some company along. Works like a charm, so long as it is another child they REALLY get along with and know well. Someone tried and true, rather than a new friend, is most likely to be the most successful.

Also, leave home the flakey friends, the wanderers, etc We always did this selfishly--if you know the other child is likely to be high maintenance, why take on an extra headache? Better to leave fun times with those kids for day long outings rather than YOUR music weekends. And then your child will have a much better experience too, and will want to return to festivals again and again. Within reason, of course. Eventually, you should send them off to do the things that interest them, not you.