21 days idea

All my life I have heard of the idea that if you practiced any habit for a minimum of 21 days that habit would be yours for life.  Recently I saw some people discussing this idea and they said it changed their life.  The challenge was to make no negative comments, use no negative language, and make no complaints for 21 days.
I would like to start with group home 1. I want to call a meeting with all the staff and girls, then I would place six- one hundred dollar bills and on the table with a purple hair tie band on it.  Any of the girls who accept the challenge would take the purple band and place it on their left wrist.  Any time they feel like swearing or complaining, or making a negative comment they can give themself a prompt by changing the band to the other wrist.
Anyone who accepts the challenge, and makes it through the 21 days will get one of the hundred dollar bills. The person doesn't have to be perfect, but has to have made a significant change in their verbalizing negative thoughts and ideas.
What we know is that unless the person puts the idea of whatever they are trying to change in their quality world it will be useless to try.  The person accepting the challenge will have to have some support, and that would be the place of the staff, administrator and social worker.  It would be really great if the girls could support each other.
I am not wanting a reward system.  I want to use the challenge theme so well used by others as a fund raiser, only we are using it to challenge children to make significant improvements in their thinking and doing, I would love to hear your ideas.  More tomorrow.