Hello Jennifer from Indiana!
No, Walt Disney World does not ask you to bring a birth certificate to prove your child is under 3.
On our first trip to Walt Disney World as a family, my youngest was about 1 month shy of 3. We never encountered any issues with her gaining the advantages of being in that age range. When we ate at buffet or family style meals we were easily accommodated for her to eat as well, without having to pay. This is not something that Disney tolerates, but actually encourages! Of course anything you buy for them at a counter service or table service meal requires payment.
I would suggest that any buffet meals, like
Chef Mickey's, that you can arrange will prove to be a great deal. Your child can get something they want, and you can get what you want, without that added cost. It is not just a money saver, but it makes everyone in your party happier! Also, make sure you check out the
Planning for Preschool Kids & Little Ones page. It is a great resource.
DOUG'S TIP: While you don't have to buy a park ticket or pay for kids meals at buffets if they are under 3, it is essential that when you make a meal reservation that you count that child in your reservation. You won't have to pay for them, but Disney needs to know how many people are in your dining party so that they get you seated at a table that is the right size for your needs.
I hope that this helps? Write us back if you have any more questions!
Have a Magical Vacation!
Doug