Having a flossing chart in the bathroom is a great reminder for kids to floss at least once a day. Have your kids use their favorite stickers to count how many days in a row they floss. Use non-monetary incentives such as staying up a little later on the weekends or an extra story at bedtime as a weekly finishing line. If they see something fun at the end of the tunnel, they'll be even more motivated to keep flossing.
You can find printable charts online, ask your children's dentist if they have a premade chart or just draw one yourself. Just make a grid with the days of the week on one side, and the flossing activity on the other.