Hi, Daphne! I know! I'm addicted to the Disney Moms Panel, too...and I'm on it! I find myself lost in all the great questions from guests and the wonderful answers from all the other moms and dads.
In answer to your great question, yes, you can either pre-order the "My Mini Surprise Cake" at Chef Mickey's or let your host/ess know when you check in. At the time of this posting, the cost is around $15 plus tax, and the small cake comes in a little plastic keepsake princess jewelry box or pirate treasure box. You can look
here at it and all the other specialty cakes available through 407-WDW-DINE. In addition, you can ask the Cast Member when you call if the specialty cakes are available at Planet Hollywood, too. My best guess is "yes," but I'm not 100% sure.
Happy birthday to your princesses! Have a wonderful day and a magical vacation!