Hi Lydia,
How exciting to be planning your family's first trip to Walt Disney World!
Mardi Gras falls on February 17, 2015. It might be crowded at Walt Disney World on this date not necessarily because of Mardi Gras, but because Presidents' Day is February 16, 2015. Since many children are on vacation from school, Presidents' Day can be quite crowded. As you move away from Presidents' Day and through that week, the crowds should taper off; however, if you could go the following week, the crowds would likely be even lower. When I am looking for predictions of crowd levels at Walt Disney World, I often visit the unofficial website touringplans.com. There, for a small fee, you can see predictions of Disney crowds for up to a year in advance. Of course, you need to decide how you as a family feel about pulling your children out of school, as well as investigate your school's policy on absences for vacations.
As to must-do experiences for your 5-year-old boy, this will really depend on his interests. If he enjoys Disney Junior Channel, I suggest that you make a reservation for Play 'n' Dine at
Hollywood & Vine at Disney's Hollywood Studios, where he could interact with characters from Disney Junior Channel in a fun atmosphere. If your son is intrigued by animals, you will want to spend time at Disney's Animal Kingdom Park taking in attractions such as
Kilimanjaro Safaris. I suggest that you order the
free Walt Disney World Vacation Planning DVD. You can watch this with your son to see what is the most appealing to him.
I would love to help you plan more; however, I am limited in the space that I have to answer. Please do write back if you have any further questions.