My current reading material, Stephen Pinker's "The Language instinct," has some interesting observations on the language learning abilities of children. He relates that on small island entrepots, merchant traders from vastly different language groups had to find a way to make themselves mutually understood. They developed languages composed of words from their various languages, but with no grammar - so-called "pidgen" languages. Fascinatingly, when children were brought up on these islands with the pidgen language as their "mother tongue", the children spoke the languages in complete, grammatical sentences, though they'd been provided no grammar by their parents or teachers. These full, grammatical languages are called "creoles".
Also: in Nicaragua, prior to the coming of the Sandinistas, there were no centralized, public services for deaf children. When schools were establishes in the eighties, the classroom instruction provided was mostly ineffectual. Nevertheless, the children (average age nine to fourteen) , on the playgrounds and schoolbuses, created their own pidgen sign language out of the languages they used with their families. Even more interesting: later, kindergarten age children were introduced into the school, and learned the pidgen language at young ages. Sure enough, with no prompting, they evolved a fully grammatical, creole sign language.
Pinker argues that, for small children, starting from age three, grammar is an instinct, something that small children do even if the language they are provided has no grammar at all. Of course, for teachers in Taiwan who have extensive experience teaching both kindergarten and fourth and fifth graders, the response will be "Of course!"