Have you read GPT-3s 175 billion parameters (words, sentences, papers, I don't care) of anything? Do you know all the words used in that corpus? Nobody has or does.
A child of a small age can listen to a very small set of things and not just come up with words to communicate with mama and papa what they learned, but they can reuse it. And this I think is key, because the language part of that is at least partially secondarily. The little kid understands what they're talking about even if they have a hard time communicating it to an adult. The fact they take creative leaps to use their extremely limited vocabulary to communicate their knowledge is amazing.