Hi there, Jessica!
Thanks for stopping by the Disney Parks Moms Panel with your question.
Celebrating a little one's birthday at the Happiest Place on Earth sounds practically perfect in every way!
We took my son for his birthday last year, and I still think about how magical it was. While you don't need to show proof of a birthday, you are more than welcome to visit City Hall in Disneyland Park to get a complimentary Birthday button. Your child can wear this during your whole visit to receive birthday greetings from Cast Members and Characters!
I recommend making a birthday trip extra special with some unique experiences like a
Character Dining meal or having your little one get a royal makeover at the
Bibbidi Bobbidi Boutique. We took my son here for the "Knight Package" and he loved his regal new look for the day which also came with a toy sword and shield. Don't forget there are also delightful sweet treats to be found in both parks and the Downtown Disney District that are perfect for a special occasion.
I hope this helps, and until your trip I wish you all a very merry un-birthday!
-Jennifer