4v4: 1-2-1 diamond
The 1-2-1 gives four players passing lanes in every direction. One player stays behind the ball, two stand wide, and one plays ahead. It suits children who are still learning to spot teammates, because someone is nearly always open.
Its limit is stamina. The forward and the back player cover a lot of grass, so rotate those two jobs often. The 2-2 square is a good alternative when you want two defenders and two attackers.
5v5: 1-2-1 diamond plus goalkeeper
The goalkeeper joins the four-player diamond without changing the outfield roles. That makes 5v5 an easy step up from 4v4, because the children already know their jobs. Only the new keeper needs coaching.
The 2-1-1 is a sturdier choice if your team loses the ball often near its own goal. It puts two players back, one in the middle, and one forward.
7v7: 2-3-1 and 3-2-1
Our default for new teams is 2-3-1; U.S. Soccer’s example is 3-2-1. See both shapes and coaching notes in the 7v7 formation guide.
9v9: 3-3-2, 3-2-3, and 4-3-1
Our default for new teams is 3-3-2; U.S. Soccer’s example is 3-2-3. Compare both in the 9v9 formation guide, with more shapes and role notes.
11v11: 4-4-2, 4-3-3, 4-2-3-1, and 3-5-2
The 4-4-2 is the best default for volunteer-coached teams. It has two flat lines of four, and each player has a clear partner next to them. The downside is that the two forwards can feel cut off if the midfield does not push up.
The 4-3-3 uses three forwards and three midfielders and suits teams with real wide players. The 4-2-3-1 adds two holding midfielders and a playmaker behind one striker, so it needs players who can read the game. The 3-5-2 uses wing-backs and packs the midfield, which asks a lot of stamina from two players.