Just getting more and more fruit into them should help slowly get them off it especially if it's the fruity, sugary kind. Easy replacement.
Chocolate is harder. People who try and quit chocolate get detox symptoms which makes someone irritable, moody and creates stronger cravings. So with kids it should be done slowly. What is their "1x" a day limit? One candy bar? Cut them to 3/4th and so on.
Hopefully they're not drinking soda..that's worse than candy. :)
Nuts/Seeds or some sort of "trail mix". Maybe they'll think of that as "candy". Maybe they're just looking for food to snack on and that could do it. Some trail mixes have pieces of caceo/raw chocolate in it so that could help replace "candy" and the taste of chocolate. Not all chocolate is that bad. Something like this - http://www.navitasnaturals.com/products/trail/trail-goji-c-c.html
Or go to Whole Foods or somewhere with bulk bins and make your own "trail mixes" where you can put little pieces of chocolate things in it and control how much is in there along with fruit/nut bits.