I had a sim as a toddler play a bunchhhh with her IF doll and it aged to a kid no problem and was walking around with her and everything and then as a child I had her play with the chemistry set a lot...
Do you use "save as" so you can go back to before your sim became a teen? If it's too long since she aged up it would probably be too much bother but if she just aged up you won't be losing much game play.