Child-like characters?

[Nestama]

Which is probably why SE have been putting them (Lalafells) in positions that you'd normally see adults in. Political leaders, accountants, etc. Lalafells were also created by Tanaka's team because of the Tarutaru in FFXI; as an attempt to get people playing FFXI to easily transition to FFXIV 1.0 (familiar races and all that). The Tarutaru also actually had children, which were actually smaller than the adults (I feel FFXIV should have done the same and not make the supposed children NPC's look exactly the same as the adults).

SE also already have a "forever young" race: the Padjal. They'll never be playable due to lore reasons, though and yes, the lore for them is actually more restrictive than the one for Viera. Padjal are only 'born' from a very specific/rare bloodline/family, if memory serves. Elementals don't just pick a rando peasant kid and go "you kid forever now lmao."

[Iscah]

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.