I think it really depends on the level of Potter fan someone is in relation to how long one can stay in the WWoHP.
Yes it is a small area but I spent the majority of our two day trip in it and really did not want to leave. Of course, I'm a true blue Potter nerd and I was completely overwhelmed with the place. I spent many hours just looking at the little details, the window displays, the displays inside the shops, the signs, the posters ect ect. There are just so many little things to see and experience inside the Wizarding World for fans that I wouldn't recommend just rushing through to the rides and through the shops.
JandKmom-if your daughter is a huge Potter fan, let her take her time and look at everything, let her listen for the noises upstairs in the Three Broomsticks, let her read the chalkboards and watch for the shadows on the walls. Let her stand in front of the windows and underneath the Owlery and at the train station and really look at it all. There are so many hidden little gems that Universal put in from the books that make it so very real. Also, even if you do the standard queue for Forbidden Journey, also take the time to do the Castle Tour line and take your time going through Hogwarts, there are so many things in there that are easily missed when keeping up with the ride queue. Same with the stores, as there are tons of display pieces in them that will make any Potter fan happy.