Hi Michelle!
Quite honestly, I think the best and most economical time to go is when older children are in school! That's because it's a lot easier to get around the parks, and a lot more enjoyable, when they are below average crowd-wise. That said, my favorite time to go is during the first two weeks of December. The weather is cool, the holiday decorations are up, and the crowds are manageable. My second favorite time after that is October though it can still be hot on some days during that month!
Since it is your first visit, you will definitely want to head to the Magic Kingdom, the crown jewel of the Walt Disney World Resort. Must things to do with a 5 year old include:
- Dumbo
- Peter Pan
- it's a small world
- Jungle Cruise
- Carpets of Aladdin
- Pirates of the Caribbean
- Monsters Inc Laugh Floor
- Tomorrowland People Mover
- Winnie the Pooh
- Mickey's PhilharMagic
After the Magic Kingdom, I suggest visiting the Future World portion of Epcot or Disney's Hollywood Studios next. If you child really loves animals, then you might consider going to Disney's Animal Kingdom before the Studios. All three parks are great, though not as special as the Magic Kingdom in my opinion, especially for young children.
By the way, since you said this is your fist time, I really recommend getting the book "The Complete Walt Disney World' by Julie and Mike Neel. It is a great book for first time visitors, because it has a lot of information and tips but is not overwhelming. It is also a beautiful book full of color photographs. It will give you a good feel of everything there is to do in the Magic Kingdom on your one day there.
Enjoy planning your fabulous Disney vacation, and write back if you have any more questions!