The day we did early entry, we went back to the hotel in the afternoon while it rained, napped, and went back at night and stayed until almost park close. We were late night in a pretty empty Diagon Alley and it was my favorite moments during the trip. My son did all the wand spells and my daughter and I sat at a table drinking butter beer. It was night so it wasn’t a million degrees. We picked up giant donuts on the way out and ate them for breakfast the next morning.
YMMV, but my DD15 is very responsible and I would totally let her be alone at a park. We let her and her brother tour without us for awhile as DH gets motion sickness and forbidden journey did him in. When she was 14 and we were at Disney we let her go to Epcot by herself an hour before close (we were staying at Beach Club, so nearby). She has a phone and so we could keep an eye on her. She had a blast.
Being at Universal with the express passes was such a relief. It was so nice to not have to have a meticulous schedule and just wander around doing things that looked interesting.