Another question...If we decided to be at USH for opening, but wanted to take a break in the afternoon, can we re-enter the park in the evening? Where and what would we do for a break?
Yes, you can leave and come back. The parking also comes with in and out privileges, just keep and show your receipt. There's lots of shops and restaurants in the CityWalk area, but not necessarily much of interest to a 3-year old. You could theoretically go to Griffith Park/Observatory. Your 3-year old might enjoy the pony rides and railroad at Griffith Park.
Even on a summer weekday, it's possible to get everything done in a relatively brief time with single rider.
We went again yesterday and just to give you an idea how our day went (isitpacked.com rated it as a "yup it's packed" day, though I think it was on the lower-end "packed" based on prior experiences):
Entered the park at 7:55, made our way to the front of the rope drop crowd. Walked with purpose to Forbidden Journey, were the third group on the ride after dropping stuff in a locker. Rode Flight of the Hippogriff 2x because we could. Was 8:20 by that point, Forbidden Journey was still only 15 minute wait. Used the restroom, filled water bottles and wandered through the shops and took pictures until 8:50.
Waited by Simpsons barrier until it was opened at 9:00, went on Simpsons. Finished around 9:20, walked over to Minion Mayhem, stopping to take pictures along the way.
Rode Minion Mayhem, finished at 9:45, stopped for another restroom break, headed to the lower lot.
Were first in line for Jurassic Park at 9:55, ride was experiencing technical problems, so they didn't open on time and emptied the line around 10:10.
Headed to Mummy and walked right on single rider (literally, didn't even wait one train cycle), while regular line was 25 minutes.
Waited 20 minutes for Transformers b/c my kids didn't want to do single rider (we would have walked right on single rider).
Went back to Jurassic Park, which had just opened back up, and waited 20 minutes in the regular line (same, kids didn't want to do single rider, but they were loading pretty fast from the single rider line)
Went back to upper lot, stopped for popcorn and restroom break
Got in line for the tram tour at 11:30, which had a 35 minute posted wait. That proved mostly accurate, got on at almost 12:00.
Tram tour finished at 12:55 and we headed for lunch at Three Broomsticks. We had a six-hour drive back home, so we left after getting lunch and another popcorn refill.
So in 5 hours, we did all of the currently-open attractions with only one use of single rider. We could have cut about 25 minutes off by using single rider on Transformers and Jurassic Park. If the new Dreamworks Theater had been open, we probably would have headed there and then tried to catch the 2:45 Waterworld show. That still would have had us done by 3:30 or so.