The paragon example, I'd say, when writing from a child's perspective in first person, in a book that is NOT for a child audience, is found in Jack Womack's Random Acts of Senseless Violence.
I'd call these stylistic points Condillacian statues effects.
1) Slang and language use that is not quite formal English is necessary if the perspective character is a child and not an adult, because they lack experience or possibility of wide reading or writing. Whatever experience they do have has a disproportional effect on their speaking and writing. And this is magnified by the fact that they imitate what language objects (words, phrases, etc) they newly experience based on analogy with the language objects they know.
It's an unintentional sampling and selection bias which results in this slang, not necessarily any desire to speak in slang where a choice to speak in slang or nonslang exists. They can't parse what is nonslang (from our perspective) nor can they, at that point, output nonslang.
We all learn by imitation and analogy. But adults have learned much more by definition than children. And it's not just the slang that's important to be authentic and provide emotional connection (because almost any reader shall remember how they were and were not when children themselves) when writing from a child's perspective.
2) Children have limited perspective, and generally do not know immediately the point of what is happening around them, for they lack experience. So the perspective character, if they are really a child, must come to conclusions if and only if their personal experience at that point in the novel justifies the inference.
Adults in contrast may simply make an observation or hypothesis or make an inference at once. (It may be true or false in the context, but the key thing is that it's made at once. There is not necessarily any lead up to it.) It depends on knowledge or experience besides that which they have through experience of the events of the story. They presumably had much experience and reading or discussion outside the story.
But if the perspective character is a child and novel discusses their life, this is not the case. The story must provide all the inputs leading to any inference or assertion of the character if the character is a child in order for it to be plausible. If the character is not plausible, although they story may be interesting, there would be little emotional connection and not much emotional impact.