Hello Nikki! Thanks for visiting us here at the Disney Parks Moms Panel. I am sure your Walt Disney World vacation will not be ruined by your situation. It is, after all, the happiest place on earth! Many people visit Walt Disney World with broken bones and they have a great time.
I love celebrating birthdays at Walt Disney World, your son is going to have a great time. If I were you, I would try to snag a dining reservation at
Chef Mickey's for breakfast before heading to Magic Kingdom Park. Chef Mickey's is located at Disney's Contemporary Resort, which is very close to Magic Kingdom. Chef Mickey's is an all-you-care-to-enjoy character breakfast. Guests can meet and take pictures with Mickey Mouse, Minnie Mouse, Pluto, Donald Duck and Goofy. You can make reservations by calling 407-WDW-DINE. I find starting out a birthday with a nice sit down breakfast is the perfect way to set the tone for the day. Not only do you get to enjoy a delicious meal but you will have met several characters, which saves time when visiting the parks. If I were you, I would spend the rest of the day leisurely enjoying Magic Kingdom. Let the birthday boy make some of the decisions about which attractions to enjoy and when to grab a snack and sit down for a bit.
I know you are going to have a great time at Walt Disney World. If you have any other questions, please feel free to ask. Have a magical day!