D.B.
I think it's always advisable to approach the coach first. Say that you understood that the rules are that kids play equal time. You have to be a little flexible on that though. If one kid plays 14 minutes and another plays 12, that's pretty equal! And the coach can't just stop the play with the officials because somebody's 12 minutes are up!
Also, be sure that you are incredibly accurate and that you know for sure how much each kid played. If they played an entire 2 quarters, that's easy to measure. So how are you measuring 70-80%? Someone is playing 2 quarters, close to 3, but a little over or a little under? Be sure you are accurate.
Then, you have to give the sports coordinator time to observe the games and take notes. Then the coordinator has to speak with the coach.
Are they recording the games? Are their films that can be viewed both to assess the playing time but also to look at how kids play and make adjustments to their technique?
Just quitting? That's a last resort. And it doesn't send a good message to the kids - it says "If you don't like things, quit. Don't try to make them better. Don't speak to the coach or the supervisors. Just walk away angry."