Hi, Linda!
With one day and a five-year-old, your best bet is by far
Magic Kingdom. It offers the most rides and entertainment geared for your child's age, specifically.
Make a list, in advance, of the attractions your child is most likely to enjoy; they might not be the popular ones, but only you and s/he know best. As long as you have a priority order, you will be able to work in quite a few. I hesitate to give you a recommendation without knowing anything about your little one, risking sending you on rides that s/he won't enjoy.
If you'd like, feel free to send me a little bit of information about your child, and I'd be glad to help out favorite character, any anxieties (fear of characters).
Have a wonderful trip!
See you later, celebrator,
-Deborah B.