Probably since there's already a perfectly serviceable word for the characters being described, that is, "children." There was no need to invent a neologism to communicate a universally-understood notion. The -ling suffix was also an odd choice. If the word was used only by Yoda, that'd be okay, he's a 900-year-old alien who never quite mastered English. But hearing it from Obi-wan and Padme is kind of strange.
I didn't feel for the poor dead younglings, but not for linguistic reasons. The PT turned me off on the Jedi so much I felt the purge was more of a mild overreaction than the holocaust I'd imagined. You know how you read about a government shooting or crackdown on some troublesome group, and you think, "Well, obviously their civil rights were violated, but they were kind of asking for it?" Yeah, that's the Jedi.