I was thinking about this yesterday. It would be pretty damn hard to explain those type of words to a young child and how to use them. By the way, is there a special term for those type of words? I think I may have heard it before but I can't remember.
I believe they are called prepositions, but not all of them. Some are conjugations, definite articles, pronouns, etc.
They learn all words that way, that's how they learn the entire language.