6 Balls has it right.
You can only go as fast as they are ready: Vest & Skis/Board on, in the water, boat passengers down. For that age: Boom & HEAVY "Chiefs" in the boat, sitting on the boom side to "feed/lower" the boom via their weight, once grabbed by the skier, have them move to the middle. Your skier is now basically out of the water (see pic). Put the boat in gear and snap a lot of pics!
This is is my 4th grade daughter, after the boom is feed/rec'd, about to put in gear. You can see how much of her body is out of the water once we shifted the weight. If you had this down, I would say you would max out at 5/hour if the girls are out in the water waiting on you.
If you have this system, you will need more boats!! My daughter & 4 friends went from day 1 boarding, first attempt, to 5" bridle off boom, to long line, back to boom for 180* turns by the end of day 1. Day 2 - no boom, and when is it my turn came up a lot.
As GregHind said, no falls the better + a lot of encouragement. Once up for 100 Yds, I made them let go and "sink/fall" in the water - to loud applause. I would get them back up and do it all over. In the car, they only talked about their wipeouts...... at about 9 mph, the dads were all cracking up.
BTW - bring plenty of fire water. You deserve it. Good on you for doing this.