Hi! I'm a single Mom of 2 girls ages, 4 and 6. I take them alone all the time. Try not to worry. Some things I do to make the trip easier, is I do get to the parks right when they open. You beat the crowds, and get a lot done before the park is really busy. If I go with both the girls, I get them the double stroller. I rent it, so I don't have to drag it on and off the bus. It's a great way to dash across the park without the little one in tow. I usually get them a drink and a popcorn, and it's also a great spot for them just to sit and rest.
I carry a backpack with the camera, autograph book and camcorder to make sure I capture all of those great memories, while keeping my hands free to hold theirs.
I also have them take breaks at the hotel in the afternoon. We usually head back around 1 or 2 pm, and they nap or swim and have a snack, then we head back in the evening for fireworks or more rides.
I hope you decide to go. It's such a special way to share time together. Good luck.