All you need to clean is the area where the teeth touch each other side by side. It won’t damage the gum tissue if you happen to go down too hard between the teeth. To avoid that, just be sure you “seesaw” your way through that contact-area between teeth, while gently pushing down.
I talk to my children about what we are doing while I’m flossing, so they will do it the same way when they start flossing on their own.
When they were between four and five years old, they started doing their own front teeth. After they were finished, then I would do the back teeth for them.