Aberrant titans can come in all sorts of forms, but thankfully that usually means we see kind of weird titans—not something resembling a toddler. That's good.
Babies are cute, you see. And a cute baby titan could totally cloud the judgement of Eren and co, like in this silly video by Animation Domination High-Def. And plus: babies eventually grow up and ask questions, right? That's bad news!
Attack on Cutest Titan [Animation Domination High-Def]