Just my opinion, having recently returned from a trip with a newly 3 year old who was 38 inches tall:
Magic Kingdom: Dumbo, Barnstormer, Tomorrowland Speedway, Winnie the Pooh, It's a Small World, Aladdin's Magic Carpets, Astro Orbiter
Animal Kingdom: Triceratops Spin, Kilimanjaro Safaris (we didn't get to ride Na'vi River Journey, but it looks like it would be good)
Epcot: Nemo, Turtle Talk, Gran Fiesta Tour, Frozen Ever After
We didn't go to Hollywood Studios, but I hear the Toy Story ride is great, and the Disney Junior Show.