Hi Pedro!
I think that its wonderful that you are taking your little princess to the Magic Kingdom! There are several companion restrooms located through out the Disney Parks. For a complete list, please see this link:
Companion Restrooms.
Magic Kingdom is my favorite park for toddlers and pre-schoolers as I feel it contains the most entertainment and attractions for them. While at Magic Kingdom, be sure to visit some of the attractions my little 3 year old daughter loves: “Under the Sea - Journey of The Little Mermaid”, "Peter Pan's Flight", "Buzz Lightyear's Space Ranger Spin", the "Magic Carpets of Aladdin", "It’s a Small World”, and "Dumbo the Flying Elephant". You can also find plenty of
character meet and greets throughout this park! There is no better feeling than to see your kids light up with excitement when they see their favorite characters! Ultimately, I gotta say that the best thing is the Wishes Nighttime Spectacular Fireworks! The music, the fireworks and the flying Tinker Bell are out of this world!
If you visit the other parks, I would recommend "The Seas with Nemo and Friends" at Epcot and "The Boneyard" play area at Disney's Animal Kingdom; “Disney Junior – Live on Stage!” and "Toy Story Midway Mania" at Disney's Hollywood Studios! Your daughter would love these!
If I can help you with any other planning questions, please feel free to ask me!
Thank you for submitting your question to the Disney Parks Moms Panel!
Yael