Give each child a piece of paper. Have them decorate it, using crayons, with items relating to brushing their teeth.
Write each child's name on his decorated paper, and tape the papers onto a wall.
Put on some music about brushing your teeth.
Tell children to brush their teeth while the song is playing. Tell them they can stop only when the song is over or when you turn the music off.
Place a sticker on the paper of every child who brushed her teeth for the duration of the whole song. Any child who stopped brushing her teeth before the song was over does not get a sticker for that day, but she can try again the next day.
Continue doing the activity for a full week.
At the end of the week, hold a "ceremony" and give prizes to the children with the most stickers. The prize can be related to teeth brushing, such as a new tooth brush or some floss. These rewards encourage them to continue brushing well.