M.F.
I've been to many concerts there and, while you can't see the details on stage very well from the lawn (ie. not able to see facial expressions), you will have a clear view of the whole stage show, you have room to move around and clear views of the screens. Being in a seat, especially for a young kid, the view will be blocked by taller people standing in front of her and she won't be able to see much of the stage or the screens over the crowd.
Go with lawn seats, mid-way back or you'll have to deal with all the people walking to and from bathrooms, snack bar, etc. walking right in front of you, bring a blanket and have fun!!