We took our 3 kids (17,15,12) last year at exactly the same time you want to go. We loved it. We were in the park 4 days and it was too much. You can get by with 2 but 3 would be perfect. Plenty for kids your age to do. Though, parents be warned, there are some vomitous rides. That's where the universal guidebook will come in handy (see below).
I'd recommend staying on property. We stayed at Royal Pacific. It's about a 10 minute walk or a 5 min boat ride. It gets you into one of the parks an hour early. And you get line cutting privileges as well on a lot of rides.
I'd recommend subscribing to undercover tourist which can give you some helpful hints and some idea on crowd sizes on the days you want to go. Maybe also buy a guidebook on universal to study the tricks (like getting in the gate and immediately running to the back of the park to get in line for Hagrid's Motorbikes). Make sure you get the most recent edition.
Make sure your ticket allows you to hop between parks. They're connected by Hogwarts Express.
Formerly Willy Wonka