Unfortunately all children are not created equals and some will crave sugar som much that Your method would fail miserably. I see this in children of my neighbours. Mine are almost same as Yours (especially older one) - they like sweets, but often they will leave them laying around not touching them for days. But if You do this with children of my neighbours they will behave like little addicts and they are like this since when they were like 3. I think this works a bit like with drugs or somkes - some people just do not have addictive personality and it seems like they are born this way (maby its some chemistry in their brains working better or sth).