Preferred 7v7 formation?

Depending on game situation and level of play, a 1-3-2-1 or a 1-2-3-1. Usually would start with the 1-3-2-1, then switch if circumstances called for it. link to Youthsoccer101.com 7v7 formation page

We play U9 with a 3-2-1 that really looks more like a 1-2-2-1 when in possession of the ball. Striker and central defender play sideline-to-sideline and stay in their respective half at all times and it tends to work best if those two players have some positional understanding/discipline. Mids should drop back in transition so we are defending with 5 across two layers. Fullbacks are allowed to come forward in attack, but we usually try to limit it to 1 at a time so we are attacking with 4 players and there is a staggered arrangement with central defender around midfield and the other fullback pinched towards center about halfway between midfield and the edge of the box. I would characterize it as a high defensive line aimed at keeping the ball in their half. It does require some mental flexibility on attack as your fullbacks, mids and striker tend to take turns as the wide players situationally and it can suffer from clumping when players lose focus.

People tend to characterize this as a fairly defensive formation (as I did before using it), but if your defense plays a high line and you strive to maintain possession in their end it won't feel like it at all. It is particularly lethal on attack when you have fast, skilled players leading the rush from the fullback position a la Danny Rose or DeAndre Yedlin or just continually serving cleared balls back into the area. With a high line it can be susceptible to counter-attacks where both fullbacks drift forward and your central defender is caught in a 2-on-1 rush, especially on shorter fields, but overall the large number of defenders has made it a comfortable formation for trying out inexperienced goalkeepers.