The difference between Lalafells and a "playable child version" of other races is that Lalafells are stylised. They don't actually look like toddlers - they have features that make us think of toddlers, but they don't look like an actual human toddler would if such a model existed in the game.

It may be a subtle distinction, but that might be enough to get a classification review board to agree that Lalafells are not children and the game is not portraying (or allowing users to portray) children in inappropriate ways. It would be harder to make that argument for a race that really look like children.