You might consider having them do a 2 week course of Chlorhexidine to decrease the bacterial load. Then when all the restorative work is completed you can go to Prevident or similar toothpaste.
Nutritional counseling might be helpful so they understand more about the sugars they are eating and maybe discuss how caries happen, ie; acid attacks and prolonged exposure to sugars.
Finally, get them chewing xylitol gum and or mints.