re: 1 day at Universal

If your 10 year old is a big Harry Potter fan, the better and more magical side is Universal Studios. It's where the wand shop and Gringotts are, and it just has the world look better than Islands of Adventure. That side is cool too but I would give the edge looks and environment wise to Universal's side. He/she would definitely enjoy the train ride and there is a spot where it looks like people disappear into the wall to get to the station just like in the movie, depending on where you are standing. The train ride is pretty cool and easy to go between the parks, and they go in between the Harry Potter worlds (you leave and arrive in Harry Potter world).

As for rides: Hagrid's Motorbike is legit and worth the wait. The rest are all on the express pass and will be quick. They are all really well done. You could very easily do everything in a day with time to spare -- there's also a castle lightshow at night in Islands of Adventure FYI.

If it were me and I was already committed to a big dollar theme park trip, I would bite the bullet and spend the money for the hopper and express passes. It will be worth it for the memories alone for your 10 year old. You can always make that money back but you can't get that time back. You may even find yourself disappointed that you did more Disney than Universal -- the express pass is EASY and the Harry Potter worlds are very immersive and well done.

Also - be sure to get some butterbeer. It's nonalcoholic so the kids can enjoy too and it is awesome.