You said that you have used the cakes before....was it early or was it the exact birthday? From what I have read somewhere (can't remember where) is that you can only use the cakes to age a Sim early only once. So, if you jumped the gun from baby to toddler then you can't jump the gun from toddler to child. The toddler doesn't need to interact with the cake. have the mom or dad click on the cake and then select who gets to blow out the candles. Then the parent will go and get that child and boom....