Every group of kids is different, but knowing nothing else, I would make it three games. First place on each game gets 12 points, 2nd 11 points, and so on. Then top 4 play in a championship round.
To keep things orderly, you can put a piece of tape on a poker chip and write the kids' name on the tape. These can be lined up on the game to keep track of order.
You could also do team tournaments, like four 3-player teams, and each player gets to play one ball for their team. With that many kids/teams, I would probably do Round Robin (each team plays all the other teams and the outcome is determined by which team has the most wins)
When talking strategy with kids I keep it super, super simple. I point out maybe one or two key shots in the game worth going for, such as the center shot on Creature (move your car) or the right saucer (snackbar, unlimited millions, etc, like. The kids who take to pinball will ask more and then I tell them more.