I think it depends upon the size and friendliness of the festival, but it is best to be a mother hen, especially with daughters. I and my brothers were allowed to wander at most festivals; we were responsible and independent kids, but more importantly, nearly everyone there knew us. My parents did the mother hen act discreetly; we were given our independence, but family friends kept an eye out for us. We didn't realize that they were babysitting at a distance, but they each made a point of saying hello as we went by, so we would know where to find trusted friends if we needed them.
Your after-dark rule makes a lot of sense. A festival may be family-oriented, but you don't know every attendee personally. It's not so much a matter of your own child's responsibility. Good kids tend also to be naive, and that gets them into more trouble than any acting up they might do.
People don't really bother 8 year-olds much, but in a few short years you will have to keep an eye out for what I call the "sharks." You would not believe how many 20-something men try to pick up 12 and 13 year-old girls, especially in the festival environment where societal taboos are not as strict. The year I was 13 and allowed to attend a large festival by myself, no less than seven young men asked me to go for walks after dark (family friends interceded each time, thankfully, by pointing out to me what their motives might really be and encouraging me to stay in public places).
Festivals are a great place to foster your child's sense of independence; but as a "festival brat" myself, I think you are right to be cautious, especially in a place with thousands of people.