There is an emphasis on flossing because we know where the real dental problems in children start, and we know what it will take to repair the damage of decay between the baby teeth. (They are only called baby teeth because you get them as a baby. They have to last until early teens.)
I wish I could tell you it’s unusual to see 4 or 5 year old children with decay between their molars, but it’s not. And it’s not just children who eat candy or sticky sweets or don’t have their teeth brushed. There are factors you can control like flossing and diet.
The best way to give your child this habit is to introduce it right from the start and BE CONSISTENT.