Adding to all the good expert advice above, I wanted to share our recent experience with our similar family.
We are a family of 4, with 7 year old girl/boy twins, and were on our first trip to UO in January. With no Harry Potter, we still couldn’t do it all in one non-Harry Potter day.
On Day 1 of our trip, we did only Wizarding Worlds of Harry Potter (fantastic!). Then on Day 2, we focused solely on the rest of USF and IOA. The parks that day were open from 9am - 6pm. IOA was a 4 and USF was a 7. We had Universal Express passes from staying on site. We had Touring Plans and UOG research out the ying yang. We entered IOA before 9am, headed to USF after lunch and were there until close.
By choice, we DID NOT do any real roller coasters (kids were too short, which we knew going in), we didn’t do any timed shows except Horror Make Up Show, and we didn’t do either of the soaking wet water rides. The Simpsons ride was closed. No playground time or really even parade watching time. The only thing resembling a break was table service lunch at Mythos. And the kicker…
Our day was COMPLETELY filled up having an amazingly awesome time doing a long list of attractions we wanted to do. (And we went back the next morning to do more.)
As I step back from the trip, thinking about our next visit, and thinking about your Q1, it’s pretty crazy to think we spent an open-to-close day doing tons of fun attractions we wanted, with a great plan and UE, while NOT doing any headline roller coasters, any Harry Potter, and virtually any shows. I think doing it in one day is going to be hard.
That said, if you really have only one day, you should go anyway. It’s too much fun.