Are Soccer lineups for players playing at the U6 level important? The players play 3v3 or 4v4 at the U6 level, so what positions they play are really unimportant in the grand scheme of things.
The young players will chase the ball at this age so learning soccer skills to gain mastery of the soccer ball is most important when teaching this age. Once they gain more mastery of the ball the shape and positions will mean something to the players because they will be able to get their heads up and see the game instead of focusing in on the ball. This will allow the coach to create soccer lineups or soccer shape more easily.
My advice is focus on teaching the skills to your young players. The players will naturally gravitate toward positions as they get older and move up to 6v6, 8v8 and eventually 11v11.
There are two schools of thought that go along with setting up a team.
I like the latter for young players because they don't understand the game yet.
2-1-2 or a 2-2-1
The team that is playing 6v6, which is 5v5 and a gk is playing in a diamond with a central player. An easy way to pick a team is pick the spine or middle positions first...goal keeper, central def, central mid and the forward. Now the coach can add the outside players into the mix.
If the coach does not have players that fit into those positions then he or she is starting from scratch teaching skill and small sided games to develop these players. The players will have to learn the game.
A soccer lineup for a 8v8 team starting out is a gk-3-3-1 so, once again make the spine strong and add your outside backs and outside mids into the mix. The kids will have a little more chance to get used to playing if they have 3 in the back.
If the coach has two very fast defenders that can control the back he or she could run gk-2-3-2. The most important area in this formation is having the outside mids track back and help out when possession changes.
This is a good balance between attack and defense.
If the coach has three very good forwards with a good holding mid he or she should run a gk-3-1-3.
This means that the team is playing with a 3 front so the forwards will stay higher up the field - forwards must work hard to defend when they lose the ball. The outside backs will have to step up and fill the spaces when the ball is on their side.
Very good shape if you have attack minded players or you want teach your players how to play attacking soccer.
Once the players are moving into select soccer the coach could be coaching 11vs11 so the players now have more to contend with on the field.
The coach can still build the team around the players that make up the spine of the team.
A gk-4-4-2 which would be a sweeper - stopper - central mid - forward
A gk-3-5-2 which is central defender - central mid - attacking mid - forward.
The trick is figure out what works for you as a coach and help your players understand what you are trying to teach.
Whatever the coach decides to do the players must develop the necessary skills to be able to play in the shape to make it truly effective.
If the players are dribbling, trapping, passing and shooting well then the shape will be more effective.