The point of view, thoughts, worldview, understanding etc. of a first person narrator should usually conform to that person's personality, maturity, culture, interests etc. That is the reason why you choose a first person narrator: to portrait that person's perception and experience. If you want a neutral voice or a voice with a personality different from the protagonist, chose a third person narration.
So if your narrator is a child, the story must be narrated as if by a child.
Note that I excluded language in my list above. While there have been attempts to reproduce the linguistic idiosyncracies or thought processes of certain kinds of people (e.g. the "idiot" at the beginning of Faulkner's The Sound and the Fury), the common approach is to use standard language even in first person narration. So a child narrator may use adult grammar and orthography, but a child shouldn't use words, ideas, or display an understanding beyond his or her age.
If, on the other hand, you use a third person narrator to tell the story of a child, you can tell that story from an adult (or any other) perspective and need not conform to age or personality of your protagonist.
Besides who your narrator is, it is also important to consider when he narrates the story.
When your first person narrator narrates past events, then of course the narrator may be older at the time of the narration than he was at the time of the events he narrates, and may display an understanding he did not have back then.
For example, when I tell someone of something I experienced as a child, I am not (when I tell it) a child and will tell the story like an adult would. I don't relapse into my child-self, just because I recount a childhood experience.
The child narrates his story:
I go to the wall and press where I'm not supposed to.
The adult narrates the story:
My parents had told me to leave the light switch alone, but I went and turned the light on anyway.
So make sure you have a clear understanding of who your narrator is and whether he or she is identical to the protagonist, not only in person but also in time.
Things get more complicated when you consider grammatical tense. An adult narrator may narrate past events in present tense:
I'm five years old when I go do Disneyworld for the first time.
Grammatical tense (in fiction) is a narrative device and does not (necessarily) indicate a temporal distance. So do not get confused by grammatical tense.