What do you do with a completely one-sided 3v3 game?

I would change something rather than watch ten minutes of goal-restart-goal. The easiest first move is usually to mix the teams for the next game. At U7 I would have no problem swapping a player between groups if it gives everybody a better football experience. During the current short game, you can also quietly help the weaker group get started — for example making sure they have a clear restart and a bit of space — without inventing complicated handicap rules. What I would not do is turn it into adults desperately trying to stop the stronger children scoring. They have not done anything wrong. Let the game finish if it is nearly done, then rebalance the next one. Short games make that much easier because nobody is stuck in a mismatch for 30 minutes.