Hi, Sarah! Children under the age of three do not require a ticket to enter any of the Walt Disney World theme parks.
I always thought the best park for little ones is the Magic Kingdom. There is so much to see and do, and I know my own tiny princesses were always the most amazed at the Magic Kingdom. You'll find that unless an attraction has a height requirement, your daughter will be able to ride with you. Things my girls always enjoyed included most of the attractions in Fantasyland (carousel, Dumbo the Flying Elephant, Winnie the Pooh), but they also loved Pirates of the Caribbean and the Haunted Mansion.
It's a Small World was a must do when I had little ones, and we always did the Tiki Room and the Jungle Boat ride. My girls loved thinking they were saving the universe on Buzz Lightyear, and when it was nap time, the Tomorrowland Transit Authority was always a good way to slow down, cool down, and help my little sleepy heads drift off.
I hope you have a wonderful vacation!