I think the OP is talking about 12 year olds not 5-6 year olds.Dont need full backs as kids go down the middle and cant cross so 3-3-2 or even 3-4-1 as they will all pile forward anyway.
You werent expecting discipline were you?
Kids can cross as my youngest son (just turned 10) plays left midfield in a 7 -a-side team and very good at crossing the ball, he is not the only one.
As for disipline i would give kids some credit, i have another son that plays so i watch lots of kids football and 99% of kids behave, it's some parents that cause problems!