I have no clue.
I got free floss for my girls from their dentist today (They are 1 and 3) and the floss says to use with children 4 and up.
I imagine you would use the floss if necessary, such as if their teeth are touching. My baby's teeth are not yet touching (other than the two she damaged) so we only floss those two teeth. We're going to start flossing my toddler's teeth, since her teeth are all touching.