What do you do if the children keep swapping bibs and teams themselves?

I’d keep some control over the groups because somebody still has to make sure everybody gets a fair football experience. I love that they want to organise themselves, so I would not shut it down completely. You might let them choose a bib colour for one round or agree one teammate, then the adult still confirms the actual groups and pitches. What I would not allow is confident children repeatedly moving themselves into the same team while quieter players get left wherever there is space. The organisation is serving a purpose: different teammates, reasonable balance and roughly equal involvement. Give them ownership inside a simple boundary. “You can choose red or blue, but these are your three for this game.” That is still a lot more freedom than adults deciding every football action.