I don't want no peas!

I can still remember my son (about 4 years old at the time I would think) proclaiming that at the dinner table. And I remember trying to convince him that what he was saying was the opposite of what he meant ("Here's the world with peas - here's the world with no peas.  Which do you want - which don't you want?).  Not sure I got through to him, but what about this business of the double negative?  It seemed so clear to me at the time - and for that matter still does - but many other languages use double negatives as a matter of course (Spanish: No dijo nada = I didn't say nothing) and indeed it was common in Middle English (I ne saugh nawiht = I didn't see nothing).  So perhaps it's just a matter of logic rather than grammar - and indeed, as I've said more than once, provided what you say is understood, that's really all that matters.  And there's no doubt that my son made his wishes clear!