Hi Stephanie,
Thank you so much for visiting the Disney Parks Moms Panel with your question. I remember taking my daughter to Walt Disney World when she was an infant and a toddler, and seeing the magic through her eyes was amazing!
You are correct in that guests under 3 years old do not need a ticket for Walt Disney World theme parks; their admission is free. I have great news for you. There are many attractions, including rides, that your little one will be able to enjoy! When my daughter was this young, she enjoyed slow-moving rides, such as it's a small world at Magic Kingdom and Journey into Imagination with Figment at Epcot. Check out this
Disney Parks Blog Post about attractions with height restrictions at Walt Disney World. This way you can see which attractions do not have height restrictions that your child can ride. The post has a chart for Magic Kingdom, with links to charts for Epcot, Disney's Hollywood Studios, and Disney's Animal Kingdom.
When you are in the theme parks, be sure to take advantage of the
Baby Care Centers. There is a Baby Care Center located in each of the Disney theme parks. These offer a quiet place to change and feed your little one, or simply to take an air-conditioned break from the park.
I hope that these thoughts are helpful for you, Stephanie! Please do let us know if you have any further questions.