Enable the boolProp testingCheatsEnabled true code in the neighborhood, then go into the family with the baby. Left click the baby while holding SHIFT. You will have a list of options. Choose the "Make Selectable" option. This will make the baby disappear from the family list, however, the baby is not actually gone. Select the baby again the same way and choose "Make Selectable" again. This allows you to select the baby from the family list. While holding SHIFT, left click on the baby, while also having the baby selected in the family list. Select "Set Age", then select "Set To Birthday". The baby will be able to grow up, but for some reason the age transition and the grow up option will not work. Only the birthday cake option will work. The birthday cake option might say that an error has occurred. Simply choose "Reset" and try with another cake. The error only rarely happens.