With the exception of the baby (the “pupa” stage) of sims lifetimes, each life stage has objects and interactions that will help them build skills for later life. Toddlers can play with the xylophone and peg box to learn creativity and logic skills. Children can play with the building blocks table and the toy stove to learn handiness and cooking skills; children can also read books to increase any skill that has a book available for it.
All of the object interactions will get your toddler and child sims up to skill level three, while reading books will get your child sims up to as high as you’ve got books in your bookshelf, potentially to level ten.
So, after teaching your toddler sims how to walk, talk and use the potty, have them play with the xylophone and peg box to max out those skills before aging them up. Likewise, have your child sims play with the blocks and read to maximize the skills that you want them to have before aging them up. Unfortunately, even though the blocks table looks exactly like the same item that is available in The Sims 2, here in The Sims 3, toddler sims cannot play with blocks, and the draw interaction to make artworks (which also helped with the painting skill in The Sims 2) was removed for both age groups.
Once your sims become teens, they can ask adults to help them learn how to drive (if you’ve got the Generations expansion pack installed), but teens can cook on the stove and play musical instruments, so have them work on those skills while they have time. Teens, of course, can also read to improve their skills.
All of this skill building while your sims are growing will help them once they become young adults because they’ll be farther along at all the skills you had them develop. With the University Life expansion pack, the scholarships that are available to sims are based on their skill levels, so there is a bonus there too.