T.M.
We switched our kids from brushing once a day to twice a day when they turned 3, and that's when I started flossing their teeth too. Our dentist suggested trying to floss their teeth twice a week at this age. The plastic sticks with dental floss on it work great, and I tell them I'm putting string in between their teeth. I just sit on the couch, and have them put their head in my lap.