Naked under the shower, thinking about childrens bodies and what defines them as such,
I had a stroke of genius.
Sorry if this isn't a question and doesn't quite fit in here but hear me out.
I've been asking what a "child" is before and been told to go to elementary school.
Turns out that person actually had a point. I mean, take your clothes off and look at your body and the answer to your problems of how to objectively enforce this without banning everything is right there.
This:

Is it a "childs body"? If you say yes, go back to elementary school, twerp.
This is a geometrical shape.
If people fap to this, they fap to curves. Wanna ban curves? Nope.
Wanna ban a certain proportion? No you don't.
You don't because digimon and pokemon are an exception and they all just so happen to lack a certain characteristic that sepparates them from a "child".
Now behold my magic as I turn this thing into a "undeniably humanoid child":

MIND CRUSH.
Childrens bodies. have. nipples.
They have 2 nipples where all humans have nipples. Also a clearly visible bellybutton but the nipples are somehow more important because they're the secondary sexual organs.
It is objectively measurable and even the most angry hillbilly god warrior can't argue against the fact that human children have nipples and a bellybutton.
I came up with this while thinking back when I saw freaking strippers in talkshows in tv at 12 am and thought about how the heck they could get away with that.
I mean, someone think about the children.
The answer: Their nipples were covered by stickers.
Thus no primary or secondary sexual organ was visible. Thus this was ok.
Then it hit me that all humans have clearly visible nipples (and a belybutton, for that matter) while animals don't.
At least not where humans have.
That said, if you think the blank geometrical shape is a "gray area", this:

Should revove any doubt, shouldn't it?
That's not even remotely human anymore.
If you disagree, kindly learn some basic anatomy by just looking at your own body.
See?
You can enforce this in an objective manner and it's beautifully simple as opposed to having a fuzzy definition of "no adult characteristics and certain proportions" and then just make a ton of exceptions.
We're talking how "society" would view things and were asking for a satisfactory explanation as to why pokemon like charmander and digimon like gatomon are "beast" and here it is.
Proven in real life even.
Can I get a cookie now?