I'm a linguist, let me try to explain. It's true that in some cultures, especially very economically developed ones, it's common for adults like parents or schoolteachers to teach kids "language". But this is more about familiarizing them with spelling, literature, writing style, and prescriptive norms (e.g. don't split your infinitives), which are all from a linguist's perspective on the periphery of "language" in the narrow sense, which consists just of core grammar. And when it comes to this sense of "language", kids learn it all pretty much with no explicit instruction: the order that adjectives go in, how to form relative clauses, etc.

Consider these sentences:

1. The angry man complained to the manager.

2. The angry about his food man complained to his manager.

3. The man angry about his food complained to the manager.

I won't get into the details of everything that's going on here, but (3) is much "better" than (2), and kids naturally learn that you need to move "angry" or any other adjective to the other side of its noun when it gets a prepositional phrase complement. No parent is ever telling their kid (except a linguist parent, maybe, since they love making their kids subjects), "Remember, Jon, when the adjective has a _complement_, it needs to come _after_ the noun rather than before it."