You may have noticed I made no comment on how the characters act, because that is completely unrelated to my comment on the art being inaccurate.
Nope, and frankly it didn't really matter if you did. Because there is little relevance to mental age and how a character looks or even acts.
Authors and mangaka are notoriously bad at emoting and dialogue for the characters of their stories, especially for children. Nearly as bad as they are about body types.
It's to the point your comment is almost more notable then the fact the artist made the kids too small.
I'm not aware of any norm regarding how characters act in regards to their age. I don't know what you've read so I don't know what "norm" you've gathered from those manga, but it probably isn't the norm over every manga released. I will however note that the premise of most isekai is reincarnation which leads to characters having a mental age which does not match their age (An extreme example would be "The Prodigy Sefiria's Overpowering Program". The MC in Kojin Tamer also does not behave like an 8 year old.) but I wouldn't call that a norm - it's a genre with specific characteristics.
Honestly can't really help ya. I started my journey in the early 80s watching Dragon Ball and Macross/RoboTech as Saturday Morning Cartoons. Much of the character design was intentionally distorted and kids were rarely drawn the appropriate size and definitely didn't act much like children. And that doesn't even count the myriad of lolibaba, traps, reverse traps, chibi, body horror, joke, anthropomorphic, ect, ect type characters. And that barely scratches the surface. There are some truely deep dark corners where the fetishist and kinks lurk that you'll want to be weary of.
tl:dr is that there few norms and assigning one or even trying to is pointless. Size and shape means just as little as the character’s mental age and personality archetype. And that's even before adding in the body snatching transmigrators, the reincarnated and thousand y/o loli dragons.