How are clubs splitting 12 U7s across 3v3?

With 12, I’d be looking for four teams of three and two pitches if the other club can support that. The big change is thinking in terms of lots of little games rather than one fixture with a bench. I’d have the groups sorted before they arrive, get both pitches going at the same time and then rotate opponents between games. You can change the threes as the morning goes on if the games are very uneven or you want different players to experience playing together. The bit I would avoid is bringing 12 and then trying to squeeze them around one 3v3 pitch. The whole benefit is that everyone is playing. It probably does need a bit more organising beforehand, but once the pitches are running it can actually be simpler because you are not constantly managing substitutions.