Thumb sucking

I have children (11, 10, and 6) who still struggle with sucking their thumbs or fingers. The oldest two are almost constantly doing it if they are not actively doing something. The younger one does it when she is tired. What are some ways we can work on decreasing or even eliminating this habit? We have tried the bad tasting polish, wearing mittens, working toward a reward, reminding them every time we see them sucking a thumb/finger. I would like them to stop the habit before looking into orthodontics.