Verity's post is easy for anyone who remembers Codspeak.
There's a difference between poor grammar and dialect. Tha' knows nowt about it. That is grammatically correct in my local dialect. My DD saying 'I could of done something' isn't dialect, its bad grammar. A small child saying 'I goed to nursery' isn't dialect, its correct small-child grammar but would be bad grammar in an older child and they'd get the piss taken out of them.