I went in July with my 5 year old daughter. We did do both parks in one day. It was fairly easy to do because of missing the "major" rides she was too small to ride. We started @ IOA as soon as they opened. (Suess Landing is a mustif your children enjoy those stories!) We used the fast pass, walked clockwise through the park and then headed over to Universal about 12:30 after we had eaten. I have been to Universal before which helped because I was more familiar with the layout, enabling us to hop quickly from attraction to attraction. If she liked something, we would do it more than one time. Then, we finished about 6:30-7:00. We grabbed something to eat and she wanted to re-visit Seuss Landing, so we hopped back over to IOA.(We had the two day/two park passes, but only used one day-saving the other for the next trip.) There were really things in both parks that she enjoyed alot, and it would be hard to pick either or. We left @ 9:00pm and did not stay for the fireworks because we were really pooped. If you are staying onsite, it would make it easier to rest mid-day. With the younger ones, you can do both parks in one day, because you are missing the "main" rides that usually have long waits. But, you have to also factor in that you have 3 children and I only had one, so that made it much easier, I'm sure! Good Luck!