This helps them gain all skills a bit faster.
Your Sim got a good jump on skills as a toddler. The Happy Toddler trait is rewarded to a child who just aged up from a toddler life stage who had all 5 toddlers skills (Imagination, Communication, Thinking, Movement and Potty) to at least Level 3. Here’s how you can earn them! Happy Toddler
Beside the 8 Traits that you can pick in Create A Sim, Toddlers also get to earn 2 Reward Traits that you will transfer in the Children life stage.