On child Lalafells looking like the adults, I consider that to be just another limitation of the character models. Maybe they reach adult height early, but they still need to get there - and we've barely seen any clear examples of children anyway. The "son and daughter" from the family in the Doman Adventurers' quest might be teenagers.

We never see 'half-grown' versions of any other races. You have dainty child-height Au Ra and Elezen, then two-metre-tall adults and nothing inbetween, when realistically there should be. We don't see children that aren't that one child-model age, and we still haven't seen any Roegadyn children. I can only assume they're there and you just don't see them portrayed in the game because they don't have the budget for it.