I do think children should be taught how to speak in more or less RP, or at least being able to pronounce 'th' when required, not dropping their Ts all over the place, and not using 'we was' or 'I done it', at least on more formal occasions.
How they may speak informally among friends and family is another matter.
Because whether anyone likes it or not, whether anyone thinks it shouldn't matter, the fact is that it can easily affect their job prospects later. Ditto spelling and writing more or less grammatical English. I know people like to say it shouldn't matter, and that only grumpy old pedants care any more, but it's just not true.
I have heard more than once of a pile of very similar CVs, qualification-wise, being weeded out on grounds of basic mistakes, and the weeders were not grumpy old pedants, far from it.