For 7u and above I recommend a basic diamond formation. It introduces the concept of a 3 tier system and promotes better spacing among players. The ideal spacing is roughly 10 steps apart so that your fullback and forward are always close to the action.
For 6u and below you may choose to have a single fullback and have the rest of the team “float”. Or you can try to assign a left, right, and center midfielder. It will be more about just introducing them to the topic of positions since all your mids will clump around the ball. If you can get the players playing fullback to hang back a few steps that will be a win.