“But the term 'childfree' is utter wank”
No it isn’t 
How about
“I hate children, and am never having them” (from 17 year old DD) instead? 
“As is describing small human beings as something unpleasant to avoid, that's basically what the term 'child free' implies”
Well, that is how some people feel about having children. Do you feel that your friend is judging you for your lifestyle choice? Assuming you have children do you think your friend feels you are defined by having children and have lost your identity? That you are ministuffstuff’s mother and not stuffstuff?
“Because comparing human beings to allergens, isn't actually very pleasant and certainly not some positive statement.”
It’s only you that have done that. DD and her friends have booked a post A level holiday to Lanzarote and have deliberately booked a child free hotel. Does this annoy you as well?
“What bugs me, is the comparison to something else unpleasant”
But to some adults the idea of having children fills them with horror. I have one child, and the idea of having 5 more would fill me with horror.
“Basically, the term implies that people who do have kids are not free and are somehow encumbered”
Isn’t that true to some extent?
“I've certainly struck a nerve with my comment 'you were a kid once.' That wasn't a dig at the decision not to have children, which is perfectly understandable, but at the blatant dislike of children some people seem to have. That seems ridiculous in view of the fact that children are not a separate species”
Lizzie48 The “you were a kid once” isn’t a valid argument. People say that to DD, but she really dislikes babies and small children. She hates how noisy they are and how dependent they are. You aren’t going to be thinking this when you are three. Some people dislike dogs, some dislike small children. Accept it.
“So do children grow up and suddenly become a different species when they wake up one morning?”
Of course they change
. Not overnight, but they grow into adults that you can spend adult time with. What a silly comment.