A main position plus a secondary one can be a sensible way to create more depth at U13 without closing doors.
Players are starting to understand roles in more detail, and some continuity helps them recognise patterns rather than meeting a completely new set of problems every match.
I would still make sure the “main position” does not become permanent identity.
For example, a central midfielder might also play as an outside defender or 10 because there are useful relationships between those roles. A striker might also experience a wide or midfield role.
I’d revisit it across the season as players grow and change.
At 9v9, I want more positional understanding than at U10, but I still want players learning the game beyond one patch of grass.
Depth first, specialisation later.
