I've participated in a few rallies (one where there were 160 kids and kept 10 pilots/planes busy for much of the day).
It's a lot of fun for the pilots who truly work at making it a positive experience for the kids and parents. It also helps to have a good ground ops crew to handle check-ins and match kids to pilots, and keep parents calm when they forget this isn't a set schedule system and get grumpy that they have been waiting for a while.
Usually the designated air boss has set up routes to fly for both faster and slower aircraft. He has also worked out the procedures with the tower crew, or what to call when for non-towered field.
Once I have the kids, I walk them to the aircraft and make sure they know the proper ramp safety items. The. Do a quick walk around explaining the flight control surfaces, key external systems, and answering questions. I am also watching to determine who has the greatest interest and would get the most in the right front seat.
Then I get them in their seats and buckle them in, explaining the seatbelt. Photo op time, then I head for my seat and buckle in. A quick explanation about the noise, and headset use. Then my new copilot reads the abbreviated checklist I have for them and we both go through the steps for engine start and taxi out.
On the way to the runway, I explain permitted talking and sterile cockpit rules. I also show off how unlike a car, I steer with my feet. I also complete the required safety briefing and explain what we are going to do on the flight.
I do the run up, let my copilot make the call to the tower to say we are ready, and then take off. After 1000 feet AGL, I let my copilot take the yoke while I shadow them and verbally coach them through the last bit of the climb and intital turn.
During the remainder of the flight, I have altitude hold engaged with the auto pilot and just provide them verbal cues of what landmark to steer to. We follow the route assigned (already inputted to ForeFlight) and work our way around the route. I also try to take pictures of everyone if the flight is stable enough for that distraction.
As we close on the airport, if I have got a really good stick, I will continue to coach them on lateral course while I handle radio calls, power and altitude. Then on long final, "My airplane" but tell them to shadow me on the controls with a light touch as we land.
Back at the terminal, more pictures, share info with the parents, and complete the kids logbooks and EAA paperwork. Hint: you can save a lot of time on the paperwork if you bring stickers with the required info, so you can just sticker and sign.
Then pick up next batch of kids and repeat until all have had their flight.
On the large rallies, flights are limited to about 30 minutes airtime. But on the few times where it was just one or two and a parent, then I'll take a bit longer.
And while flying the kids is a blast, even more fun is afterwards when I have had the chance to take a parent or adult volunteer on their first small airplane ride. Always a good time to bust myths and change their perception of what general aviation is all about.