I have pretty much the opposite experience.
Sims 4 toddlers are the best Sims toddlers, imo. I love the way that they essentially grow up incrementally via their skills, and I've personally been able to get every toddler to level 5 in all skills that I've wanted to do so, even on normal lifespan with MCCC set to make skills harder. It does require making the household pretty much revolve around the toddler though, so I have to decide early on if we're going to go chill and get the level 3 trait or go for the gold. For me, all of that is fun!
Children have several child-only accomplishments and activities as well, at least if you have Parenthood and Kid's Room stuff. If they finish all of the aspirations I want them to do and get their A in school, there's still character values and scouting/drama club to max, as well as purely fun things like void critter training and puppet shows.
Teens, on the other hand, I find to be pretty meh. There's nothing much that makes them feel unique compared to young adults, but they can't really get started on careers or building a family, etc. Once they've tied up any loose ends from childhood and gotten their A, they just feel like a pause button between child and YA. Even skilling on them feels less useful with University now, since classes will build the skills for their career anyway and it's not like university is expensive enough to worry about getting lots of skill scholarships.
I know a lot of people want improvements for babies and elders, but I want them for teens the most.
