Hi Heide,
How wonderful to bring your little ones to Walt Disney World for their first visit! Be sure to get free buttons for your children at Guest Relations at any of the Disney theme parks. Both of your children can wear a "First Trip" button, and on his birthday, your son can wear a "Happy Birthday" button. Although no extra magic is guaranteed with these buttons, you never know when a little pixie dust might be sprinkled on your vacation. I will never forget the look of wonder in my daughter's eyes on her seventh birthday when our server at The Plaza Restaurant in Magic Kingdom brought her a huge birthday cupcake! It is also really fun to hear cast members and other guests wishing your little ones a happy birthday or happy first visit.
To make his birthday even more special, you might consider having a Disney-themed birthday cake made for your son. You can customize one by calling the Disney Cake Hotline at 407-827-2253. You also might consider ordering him a custom gift from
Disney Floral and Gifts. Once you have perused their website, give them a call at 407-WDW-GIFT (407-939-4438) and a Dream Maker can work with you to customize a gift for your son.
As to must-do items for your first trip to Walt Disney World, that will depend a lot on your unique children. If your children really enjoy princesses, you should absolutely see Enchanted Tales with Belle at Magic Kingdom, a retelling of the Beauty and the Beast story with audience participation. Or, if your children love the Disney Junior characters, be sure to go see Disney Junior - Live on Stage!, where children can sing and dance, at Disney's Hollywood Studios.
I would love to help you plan more Heide; however, I am limited in the amount of space that I have to answer. Please do come back and ask if you have any further questions!