Yes, children master a great deal of their grammar before they enter school (most of them), but then why teach them if it weren't to give them the real understanding of the why, such and so... and why do children find it so hard?
Because those children speak a kind of Dutch (a dialect or sociolect) that differs in its grammar from the Standard Dutch preferred in certain jobs and situations. They speak this way, because they acquired the grammar of the dialect spoken at home and in their neighborhood. If you really want them to speak proper ("correct") Dutch, you should remove them from this improper environment and put them in one where only proper Dutch is spoken. Then they will acquire proper Dutch without any need for teaching them anything.
Many children and adults make all sorts of errors (both according to the grammars of their learned dialect and of proper Dutch because they mix them up and/or have not really learned how to code-switch between the different languages they know.
**Ha! example!!! Universities here complain that a too large part of newly entering students is no longer able to write correct Dutch.= correct
Universities here complain that a too large part of newly entering students are no longer able to write correct Dutch. = incorrect.
Yes, I would call this a grammatical error. You see this sort of thing all the time. I am not sure that it is generated by the writer's faulty grammar or by some other problem. It's kind of a clunky sentence, too. I'd've rewritten it as:
Universities here complain that many newly entering students cannot write correct Dutch.
It's the sort of error that probably cannot be corrected by teaching writers some grammatical rule of subject-verb concord (which I would argue all have in their grammar), but to teach them how to edit their own writing or another person's. And you teach them that in high school, although, more and more, this sort of lesson is being pushed off to universities.
