In 5 v 5 flag football, I found that a base 1-3-1 zone worked very effectively. In 6 v 6, we tended to use a 2-3-1. Here is how each works: In the diagram below, I have not drawn the individual offense player positions, just indicated generally where the offense is lined up. The red x’s are the defenders.

As a general rule, a zone defense is the best type to use in youth flag football. Many coaches try man-to-man and fail, because there are so many ways to beat a man defense. In certain situations, you may want to deviate from this, using either man-to-man or a zone +1 type coverage, but your primary base defense should be a zone defense.