I think it also depends on what time of year you are planning on going. During the peak busy season the lines are pretty brutal so you might not be able to do as many attractions as you want. If it's a non-peak season, I would think one day is enough, but once you get there you may realize that there is a lot of other rides and attractions that might interest them. And if you are spending the money to go, you might as well get your money/time out of it and do as much in the parks as possible. For that reason, I might consider a two day ticket. Some other things to consider:
- You might want to get a park-to-park pass, which is the only way to get on the Hogwarts Express, which is both a ride and a mode of transportation to connect both halves of the park.
- If you get a wand from Ollivanders, there are lots of cool interactive wand spots around both sections of the park, and those are things people tend to do over and over because they want to see if they can "improve" their magic.
- Prepare them ahead of time that they might not get chosen to do the Ollivander wand experience. It sucks but they only pick one person and you don't want a kid to walk out in tears because they thought they were going to get to do it
- Make sure to check their height before going! The Forbidden Journey ride is really cool but it does have a height limit, and I've seen too many kids also walk away in tears when they are excited to ride but find out at the last minute they are just an inch too short.
- Hagrid's Motobike ride is a bit more of an intense coaster, so be prepared for them to be too short or maybe a little freaked out when they see the cars whiz by.
- And like I said above, there might be other attractions in the parks they might be interested in. For seven year olds, the Despicable Me, Cat in the Hat, and Spider-Man rides are pretty popular. Islands of Adventure has a couple of cool (literally) water rides that become really appealing in the summer heat.