I would plan on at least 3 days/2 nights. That's how long we spent our first time and we saw a lot, but not everything. Part of it depends on the ages/heights of your children. If they are babies/toddlers you will need to do the baby swap for a lot of rides and that slows you down a bit. If they are at least 42" tall they can go on all but the thrill rides and you'll get more done. It also depends on how long, if any, of a midday break you want to take. We went back to the hotel for a few hours every afternoon. This visit we are only going to Universal and will be there for 5 days/4 nights.
Oops, I just reread that your kids are 5 and 2yo. so you will be doing the baby swap. I would plan on at least 3 days. Staying onsite you will have the front-of-the line access and will get a lot done. The rest you will do next visit!