Hello Samantha!
We will have a four year old and two year old on our next vacation! Our last vacation they were three and a little over a year. We found that Magic Kingdom and
Animal Kingdom were their favorite parks.
Magic Kingdom Park is full of rides perfect for toddlers and preschoolers! It's also a great place to get autographs and photos with their favorite characters. It will definitely be the park you'll want to spend the most time in. We typically devote at least 1.5 to 2 days to MK on our trips.
Animal Kingdom is great because there is a lot of room to explore. My kids could spend hours digging around
The Boneyard. It's a great change of pace from standing in line for rides and sitting down. I love it because it wears them out right before naptime! We find that at this age, AK is a half-day park because our little ones can't experience the mega attractions like Expedition Everest.
If you are planning on purchasing the Park Hopper Option with your tickets, you could use the other half day to visit
Disney's Hollywood Studios, which features a great Disney Junior Show, The Little Mermaid, Beauty and the Beast, and Fantasmic!.
I hope you have a magical vacation! Please come back and see me if you have any other planning questions!