Studying art styles like handwriting, Billy's is sloppy, tilted, appears like he's trying to create a charming cartoony style but it just comes off creepy. There's something gross to me about his linework (and I don't mean because he traces from his literal photo collection of young children, which is creepy no matter the art style). I knew someone who had a book on this kind of thing, I gotta study up on it.
If I may throw forth a naïve (or at best semi-educated) take on it:
Large head, small body is fairly common in a lot of cartoons to allow the face to do a lot of the work, but large head, small body is also a signifier of neoteny, or babylike proportions.
If it's the former, then one would expect his faces to signify an adult mentality. Popeye, Jon from "Garfield", and Betty Boop have big heads, but they're also drawn to be adults. Most cartoons with "teens" shy away from the neoteny and go towards the broad-shouldered beefcake, the wasp-waisted beauty, the skinny dork, and so forth. Like nobody thinks that Velma in "Scooby Doo" is an actual underage child. Physique as a signifier of where they are in the social pecking order. Then you have kids like "Family Circus" where the neotany is visible and intended.
For this one, they are drawn like they are young. Like very, very young. And there's nothing to go against that.
Like I was unfortunate enough to see some real CP during the last wave of attacks. A naked girl with her legs spread. Based on the size of the head to the body, she might have been four.
Billy's art has those proportions.
Big heads, chunky bodies, shapeless sacks for clothes.
Huge plausible deniability about whether these people are supposed to be 8 years old or 18 years old or some other age.
Gross.