When you say "it thinks that it's Maryanne who I want to have it for", what exactly do you mean by that? Is your child sim unable to blow out the candles, or some other problem like that? I know that wedding parties have to be organised by one of the people getting married, but it shouldn't matter for birthday parties, since parents can throw parties for their infant sims, and you can have more than one sim blow out the candles of a cake and age up. Generally all you need to do it click on the birthday cake and select the sim you want to age up from the menu. Even if you're currently controlling the aunt, a drop down menu appears and you can select another sim - eg, the child - and they will be the one to blow out the candles.
