Well, I know there was at least one instance of a lalafell NPC specifically being identified as a child by his nameplate (as part of an Echo-vision during one of the GLA class quests), but physically he didn't seem that different from any other lalafell that were clearly adults.
As for the 'younglings' thing, I remember hearing the term used in the Star Wars films (specifically Episode II), which I think predates Martin's stories. But I could be wrong on that.
