Good English
No such thing?
The problem is that nobody quite knows what it is.
Whatever variety of the language someone has in mind, either as an ideal or as something to be disparaged and avoided, its grammar is enormously complex and subtle.
The grammar of ‘non-standard’ varieties of English have not been deeply explored. One of the rare instances of such an exploration was by Alison Henry of her own native variety of Belfast English. The divergences were deeper than she had expected. There is no reason to think that the same does not hold for the many unexplored varieties.
One of the things which makes learning to talk so hard is that the evidence that children get, what is known as ‘the primary linguistic input’, is not good evidence. Only some sentences are finished. Many are unfinished. Of course, the child does not know which is which.
What children learn may be a new English.
By three, most children can, and do, say things which have never been said before in the whole of human history.
By three and a half. most chidren are understood most of the time by people who do not know them.