Actually, adult lalafells and child lalafells have the same models. So lalafells do look like children even in the ffxiv universe. Even their emotes are quite childish. Does it really matter, though? The adult lalafells are treated as adults, and they act like adults as well.
If a new race or character model option came out, and it happened to have the same stature and physique as Alisaie or Alphinaud for example, I think that would be fine. If they are an adult, and as long as they are treated as an adult and behave like an adult, I don't see the problem with the character being playable. If people have a problem with this, but think lalafells are ok, then it's just fake outrage.
edit:
Just fyi, I am
not asking for younger versions of the races to be available. I just like characters that are on the smaller side, and that are also slim.