Hi Dana!
Oh boy, I wish I could see your little ones' faces when they see Walt Disney World for the first time! Those ages are precious and I know they are going to have an amazing time!
I can completely understand your conundrum because
Disney's Magic Kingdom offers so much for
little ones! From meeting
Mickey Mouse at Town Square to riding the
Prince Charming Regal Carrousel to splashing at the
Casey Jr. Splash 'N' Soak Station, there is no shortage of joy-making activities!
However, I also think there is so much at
Disney's Hollywood Studios and
Disney's Animal Kingdom that your littles will enjoy! Hollywood Studios is home to
Disney Junior Play and Dance!, meeting the
Disney Jr. Pals, and Toy Story Land! My son was obsessed with Buzz Lightyear when he was 2 (and beyond!) so visiting Hollywood Studios was a "must-do" for us. If your little ones like animals, Disney's Animal Kingdom would be great. They even have an
Affection Section petting zoo!
Since you have the
park-hopper option, you can visit one park in the morning and a different park after 2 pm. Since you have four days in the parks and a park hopper, I definitely think that you can fit in all four parks. You can also choose to visit Magic Kingdom twice and only go to Animal Kingdom for a half day.
Make sure to take a lot of pictures and consider purchasing
Disney's PhotoPass. I treasure the photos I have of my kids during their trips to Disney and I know you will want mementos that will last a lifetime.
Magically,
Stacy