The typical characteristics of child-like appearances is proportionally large head compared to the rest of the body, round head, and large eyes.
It also happens that having a large head with large eyes is a common method to make it easier to express emotion for objects which we know is fake. For example, the two main robots in the movie wall-e utilize this a lot, and yes, one can interpret that as if both robots are children as they do have more human child like proportions than human adult proportions.
The only real way to know the age of non-photo non-realistic image of a character is in context or if the author spells it out. If we were to guess the age of Donald duck we would base it on that he has a job, has nephews, and wear a time appropriate hat for adults, but also that his head is slightly less round compared to Huey, Dewey, and Louie.
It leads to inconsistency, which leads to people creating content that is well within the guidelines deciding to not publish to Patreon as they can't guarantee whoever is enforcing the rules that day won't arbitrarily decide otherwise.
(Lastly, hentai is obviously a very different thing than anime. It is clear that Patreon is targeting anime, as well as hentai.)
> They're not banning anime. They're banning the fetishization of young girls, which has been against the community guidelines for quite a while:
> However, we have zero tolerance when it comes to the glorification of sexual violence which includes bestiality, rape, and child exploitation (i.e., sexualized depiction of minors). This is true for illustrated, animated, or any other type of creations. Patreon reserves the right to review and remove accounts that may violate this guideline.