Volcano Bay is open year round but during the winter months the park opens later and closes earlier and if Universal doesn't like the climate they may not open some days. I went for a week in July of last year without express and was able to do everything I wanted in two days. If your kids are a bit older you can take advantage of single rider (Hulk, Mummy, Gringotts, Men In Black, and some other rides). Also, if you are staying onsite take advantage of early park admission you can do a lot in that first hour. For example we did Minions, Gringotts and Mummy before the park officially opened.