Hi there, Donna!
With a confirmation number for your appointment at the
Bibbidi Bobbidi Boutique and valid
Magic Your Way tickets, you will be able to enter the
Magic Kingdom any time after 7AM, which is when the Park opens for Guests with early dining reservations. You must have both the confirmation number and the Park tickets for entry.
Bear in mind that you will be restricted to the front of the Park and none of the attractions will be open (unless you are traveling on a day with an 8:00 AM opening time or morning
Extra Magic Hours for Resort Guests). You will be able to get lots of great shots of Cinderella Castle
Your grandson is welcome to wear whatever he would like for his
Knight Package (as seen in the image on the Disney Parks Blog, this young Guest is in street clothes)! You might pick up a prince costume or create one yourself if you are that crafty, but please remember that Florida can get awfully warm and humid during much of the year.
Have a wonderful time!
See you later, celebrator,
-Deborah B.