Hi Sherly,
Thank you for visiting us at the Disney Parks Moms Panel! You've brought a difficult question... I love all four Theme Parks equally for their unique storytelling abilities, thrills, entertainment, dining and more.
At Epcot, your family will likely enjoy The Seas with Nemo & Friends, Turtle Talk with Crush, Living with the Land and Innoventions East & West for hands-on fun. If your prince is 40" tall, you could also enjoy Soarin' or Test Track for spectacular thrills! Walking the World Showcase will be such a treat for you and your mom especially, as each represented country will fully immerse you in their native culture. Epcot requires quite a bit of walking, so my suggestion may lend towards Disney's Hollywood Studios. Here, you'll find family favorites like Muppet Vision 3D, Lights, Motors, Action! Extreme Stunt Show and Toy Story Midway Mania! (my kids' favorite attraction in all of Walt Disney World). But I think the biggest hit with your li'l Swashbuckler will be a visit to Disney Junior - Live on Stage! He'll likely enjoy watching Doc McStuffins, Sophia the First, the Mickey Mouse Clubhouse gang and Jake and the Never Land Pirates in a brand new show debuting this past February in Animation Courtyard. You may decide to end your magical evening with the ever-popular Fantasmic! nighttime spectacular.
I hope my suggestion was helpful, Sherly, but I think you should also check out the website and filter "
preschool attractions" by Theme Park. You can do so easily by clicking the provided link, and this way you'll be able to determine what will work best for your family!
Wishing you a MAGICAL vacation!