Your DD can get a "It's My Birthday" button for free. Short of that anything else is Pixie Dust and at the whim of the CMs. You can notate it on your dining ADRs and you may, or may not, get a free cupcake, cookie or dessert. If staying at a WDW resort she may, or may not, get a signed card from Mickey on the actual day of her birthday. Most CMs will wish your DD a happy birthday, some guests might even with her 'Happy Birthday". There are no discounts on the tickets, dining, extra activities, etc.