I've done both parks comfortably in 1 day with the unlimited express pass from the Universal hotels, but if your kids are going to spend a lot of time in the Harry Potter area, I would plan for 2 days
Two days. In addition to HP, your kids are gonna like the areas for the younger crowd in each park, too ..well, maybe not the 8 year old...There a lot more to do than just HP. Definitely learn what the little one can do in terms of height before you go. I think you'll want to hit the animal show, for sure, it was surprisingly cute.