Went for DD's 10th b-day and inquired at Guest Relations/Services/whatever and all they had was a sticker that they wrote her name on. It fell off about a minute later when she walked thru a cooling mister thingy. We didn't eat at any sit down restaurants so maybe they do something there (cupcake or ice cream?) I dunno. She did wear her "Happy Birthday" crown over her hat and people who noticed wished her a happy birthday. I was a little disappointed that we didn't have some kind of b-day specific souvenier (lost the sticker after it fell off) but we still had a great time!
BTW: we stayed at the Residence Inn Sea World and when we mentioned her b-day they delivered a basket of goodies which included a small Shamu plush & dolphin snowglobe, keychain, candy, etc. and handmade b-day card to our room. THAT really made her feel special because it was completely unexpected and she shared the loot w/ her cousins.