I think you can unlock another Sim with Simcash at a fairly low level (sorry, I don't know the exact one), and then when you have an aged up child, click on the family portrait, which will take you to your generations gallery. Tap on one of the dark, empty silhouette slots and a little box should come up that will have the two options of "Create" and "Promote". Click on promote, then tap on any one of your children (a little animation should ensue), then proceed through the process and then after adjusting their looks (although you don't have to), the child will became a playable adult Sim.
@waekamania this is the sims mobile not the sims 4 PC game. Mobile does not have a cake to age up toddlers or sims. You have to do family events with toddlers in mobile to have them age up..
You can now give your children and toddlers a makeover Tap on the child and select 'makeover' at the left bottom.