The "all nonbinary terms are childish" rhetoric feels a lot less like "childish terms are getting applied to nonbinary people against their will" and a lot more like "I think all nonbinary terms are childish because nonbinary people are inherently childish to me."
Anyway "enby" is a perfectly reasonable shortened form of "nonbinary," y'all just have sand in your underpants.
And before anyone comes at me:
If you are nonbinary and you do not like being called "enby," fine. I'm not here to tell you to use a word you don't like. But maybe unpack the reason you feel the need to talk about how "childish" it is. I have seen other nonbinary people go on entire tirades about how it's "offensive and wrong to use it as a general term," as if it's some kind of slur.
It is literally just a shortened form of "nonbinary." That's it.