Julie,
Let me take this in a few parts.
Character Experience: No, there is no discernible difference in the character experience at
Chef Mickey's when comparing breakfast and dinner.
Transportation: If you think this is a good destination for a celebratory breakfast (and I do!), getting to and from it on a Magic Kingdom day will be no problem. It is just a short walk (less than 5 minutes) from the Magic Kingdom bus depot to the Contemporary. So really all you need to do is take the bus from the Animal Kingdom Lodge to the Magic Kingdom, then walk over to the Contemporary. When you are done with your meal, you can either monorail back to the Magic Kingdom or walk. The walk will actually take a lot less time!
BOTTOM LINE: For a 5-year old, I think a breakfast at Chef Mickey's is a great way to celebrate a 5th birthday! I think breakfast is better than dinner, for many reasons. It is less expensive, the foods at breakfast are more suited to a buffet (to me anyway), and I just like the atmosphere more at that meal.
Write me back if you have any follow-up questions!
Have a magical (and delicious) vacation!