Oh. This is one of the reasons I hate the anime culture..
My favorite anime is One Punch Man. I just like the vibe of the character and his depressive outview. I sympathize with Saitama, such a well written character. However, go to the onepunchman subreddit and god damn, most of them are drawing children in minimal clothes. What the fuck is wrong with everyone.
Essentially. It's kind of a fuzzy definition, but it refers to characters who, even if they aren't explicitly stated to be children within the canon of their story, are obviously meant to look/act like children. It's a controversial subject, in part because it's somewhat subjective, and in part because pedophiles don't like being called out
Animation is so weird when it comes to ages. You get the examples you're talking about where not only does the character look like theyre eight, but act like it too.
The you have things like Brock from pokemon, who was running a gym, was pursuing his pokemon breeding career, traveling the world, and is supposed to be 15?! Ash is supposed to be 10!
Edit: Like could you imagine gangs of 10 year olds roaming the streets assaulting people with animals?!
The seemingly nonsensical information in the pokedex, like a pokemon that's hotter than the sun or size to weight ratios calculating out to absurdly high or low densities, actually makes more sense when you realize it's all written by unsupervised ten year old kids who are pulling numbers out of their asses.