What would happen if I aged up toddlers b4 maxing their skills?

Check out this link for some good toddler info! https://www.carls-sims-4-guide.com/parenting/toddlers.php

By maxing out skills, you can gain a headstart in child skill building and earn some extra traits that speed up future skill and career gains. You don’t acquire any negatives if you don’t max out skills - the Sim kid can still talk and walk. There’s really no dramatic downside to not maxing out skills, you just won’t earn the extra bonuses.