I have taken my daughter who was 7 at the time to an all day/night festival headlined by Metallica with many awesome bands. It's not unsafe to take your kids but you do need to cover their eyes for sure when you see things ahead not good, avoid the crowds in the back of the venue (a no brainer), and buy seats if possible. The event I took my kids to was Genr'l Admission.
You could just attend the latter half of the day and go straight to your seats. Bathroom runs may create opportunities for unexpected things to run across your path but that's likely like someone said, a zealous teen who may have gotten paint and isn't wearing a top, showing off like an idiot. There'll be people buying beers, but mainly in the well lit and vendor areas there's nothing to worry about. You won't want to drink though. I wouldn't go without a male if she's going to keep things safe. I was with my guy at the time and my two sons, so we were well guarded at all times. And, as far as what you've explained here, if your instinct is not to take her, I would be honest with her and tell her that you're not sure about the kind of crowd that's going based on feedback from others more familiar with the concert, and that you' be afraid for her safety. And, like I told my now 9 year old daughter, we're definitely going to see Ozzy when he comes round later this year in concert together, and she can take that to the bank.
I'll believe it the day Ozzy sings a real love song. ::animatedlaugh