Christy,
While prices can change over time (especially during the holidays), at
Cinderella's Royal Table the per adult price is typically around $68, and the per child price is around $42. So for your family of 3 that adds up to around $180.
NOTE: These prices
INCLUDE tax and gratuity, which is not the norm. So budget accordingly!
As for birthday celebrations, when my girls were there for a birthday last year they got a lovely card and a nice surprise at dessert. It may not seem like much, like not getting the birthday song sung to you, but the real show is the princesses, so it seems perfect.
BOTTOM LINE: Plan for around $180, and get ready to meet the royalty!
Have a magical (and delicious) vacation!