I would make a note on your ADR and tell them again when you check in at the podium.
If it is a restaurant that plans to do little birthday surprises, they can plan on having X number of cupcakes (or whatever) on hand that night. I am SURE that they always have extra, but it never hurts to make an extra note.
(I am sure that there are extras because our DS got a cupcake the day before his birthday at Prime Time just because the waiter knew his birthday was the next day - it came up in conversation. Before anyone flames me, I did NOT tell anyone it was my son's birthday, we were clear that it was the next day. Though we didn't turn down the cupcake)