Haven't got first hand experience but do you think it might be a case of "shutting the gate after the horse has bolted?" Kids have been weaned on and fed a steady diet of junk and to a lesser or greater degree are going to be addicted to it. Therefore, short of any initial novelty value, they are resistant to the healthy alternative? On that basis I think we "reap what we sow". Force feeding the healthier alternative likely won't work, perhaps they need weaning on to it.
Of course, I might be "barking up a gum tree" completely.
