in my experience, children have several 'speaks'. They have playground vernacular and accent, used among their peers and family. But they generally know how to modify their speech in formal situations. They can even adopt a very workmanlike RP pronunciation when required (job interview, presentation etc) Most young people aren't daft, and know how to adapt their communications skills to the situation.

I was born in West London and actually spoke in a rather Cockney-ish accent, with plenty of glottal stops and dropped aitches, much to my father's disgust. At Uni in Edinburgh, I started to speak 'Edinburgh' (and 'posh' in tutorials). Now I've been in Norfolk for forty years, and at first picked up a very convincing 'Naaaridge' accent. We're out in the sticks now, and I notice I'm more 'Norfolk' these days. My neighbour says he can't tell I'm a furriner. While visiting Edinburgh last year, I found myself slipping effortlessly into the old accent while chatting to a taxi driver, much to my husband's amusement. Words like 'wee', 'aye' 'hen' etc came so easily, it was fascinating.

My point is, if one can adapt and modify one's communication skills as the situation requires, one can't go far wrong.