There is increasingly stuff in the news about the bad effects of excess sugar and obesity. Kids see that message day on day so maybe you could gently raise the potential risk of diabetes on the back of that? Even 'subliminally'. When I was a youngster I was the type who hated to be 'told' anything, rather relying on things I discovered for myself. If you somehow manage to get the youngsters talking about it, the massage may begin to sink in.
Having said that it's the parents who perhaps need to be targeted more than the kids - it's they who buy the stuff after all.