They either grow up on their birthday or you get a cake either from the grocery shop or the buy mode (F2) and then let a big Sim click on the cake then press "Blow Out Candles" and then a list will come out and the names will show up of people who are in the lot, then you press on the baby that you want it to grow up then the Sim that you chose to click on the cake will pick it up and put it somewhere on the floor next to the cake, then the baby will age up and become a toddler.
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Only the baby can grow up. Which is 3 days. The boy or girl Sim(child sim) can't grow up.
Babies growing up has nothing to do with ambitions. They just grow up like any other sim. If they're not aging at all then make sure you have Enable Aging on, but they should just grow up on their own.
In the sims 3 it is impossible to kill a baby.
Six in total, it take 3 days for a baby to become a toddler and 3 days for a toddler to become a child.
It takes about a week (in sim time)