Yeah, I've noticed that. The toddlers and kids are such hell spawn but then when they are teens, they are smokin' hot. Literately, when sims walk by, they do the whistle. Once, one of my sim's older brothers girlfriend did that every time she walked by him. But not with her actual boyfriend.
The weird part is, when I'm creating a family, I could just get 2 random sims out of the sim bin then play with genetics. The toddlers and kids and teens that come out of them are actually decent. Like pretty playable. But then, I'll take ages making a perfect couple, symmetric features, nice hair, not too different-not too twin like. Then I'll play with genetics. And oh, my great god. I already explained how ugly they are before, so I won't again. It just frustrates me.