How do you keep track of which child has played which games?

I would keep the tracking very basic. You do not need a stopwatch for every child. A simple grid with the players down one side and each round across the top is enough. Put a tick when they play that round. That lets one adult see quickly if somebody has accidentally missed two games or if the same eager child has played every round while another took a break. Do the planning before the first game and only change the sheet when the groups actually change. I would measure opportunities in games rather than exact seconds. With six-to-ten-minute matches, one extra round matters more than whether somebody played 34 minutes and somebody else played 36. The system should help the football run, not turn a U7 festival into administration.