It's relatively rare to find fictional children who act like real children. More often than not, fictional children talk and act like miniature adults. Oftentimes, this is a deliberate artistic choice, which may either be played for laughs (as in Rugrats, The Simpsons, or South Park, for example), or used to show that there is something seriously wrong with the child in question (as in The Umbrella Academy, and many anime series). On the other hand, some creators seem genuinely unable to fathom how children think and behave, and so write them behaving like adults by default.

What are some examples of stories that portray children this way? What, if any, differences are there between stories that portray children acting like adults for artistic reasons, and those whose writers simply don't know any better? What effects, if any, do fictional portrayals of unrealistically-mature children have on how people view children in the real world?

Oh, cool topic. Interestingly enough, the first examples I thought of regarding children who don't act like children, are from PBS (whose programs are all geared toward young children). Arthur, one of the longest runners, is an example. You'll also find some of this in older shows like Wishbone. Outside of PBS, the phenomenon exists on networks you mentioned, like Nickelodeon, or Disney Channel. Sometimes it works great (see the older show Fillmore for an example of unchildlike behavior as an artistic choice). Other times, the kids just act like brats (i.e., Hannah Montana). – Stephanie M. 4 years ago
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Fictional children acting in unchildlike ways