Do you let them switch teams in the middle of a festival if a game is poor?

Yes, I would change it sometimes even if the score is close. The score is only one clue. If one child has barely touched the ball for two games because the other two are constantly competing to do everything, that is a reason to alter the group. I would not change them after every small problem, though. Sometimes the learning is in working out how to share the ball or help each other. Give the group a little time. If the same picture keeps repeating, swap one player and see whether the experience improves. That is the benefit of short games. You can make a small adjustment without announcing that a team has failed. I’d use team mixing to change the learning environment, not just to correct the result.