a lot of writers are great at writing adults bc they see adults as people with 1. motivations 2. a past 3. a concrete ability to work towards a goal. all of these are things children are presumed to lack, for one reason or another - adults see children as impulsive, distracted by every shiny thing, when not all children are like that, and the impulsiveness of certain children still does not preclude underlying desires that are consistent to them; children have a way higher past:person ratio than the average adult! week for week, children are more profoundly affected by regular experiences because it is all new to them. everything sticks, and they remember. what they don't remember and what they do can be random at times and less random at others, but each child has (again) an underlying logic for what gets held onto harder than other things; children's lack of ability to work towards their goal is in large part why they are so prone to meltdowns. if you couldn't drink a glass of water when you're thirsty - if you had to get someone to help you each time, you'd end up pretty pissed every time they dragged their heels as well. and if that was true for everything you needed? [a/n: i'm aware this is reality for many disabled people! which is why adultism and ableism are so closely entwined as well]. but children ARE capable of working towards many of their goals in their own ways, whether those are emotional or physical goals.
but if you don't see children as people, though, all of those things fall apart, and you end up with the...idea of what role this creature is supposed to fill in the lives of people who look at her. this is inherently dehumanizing, make no mistake, it makes the child something that never appears to experience emotions that don't fill that idea or diverge strongly from that role, which in turn makes the character flat and uninteresting. there's no reason for us to care what happens to her, narratively, because the only thing we do have to care about is what happens to the people who care about her. she's not a character, she's an object that furthers the emotions of the other characters. she's a morality pet, or meaninglessly cute, or an aesop. there's nothing outside that for her. you could replace her with a puppy and literally nothing would change.