I have been doing some small soldering projects with my daughter and her friends for a few years now. They are now 8-9 yrs old and did their first soldering project around 2 years ago (we do other non-electronics projects too so they have probably done 3 one-day solder projects in that time).
I would say that at their current age and experience they are now mostly capable of soldering components to a board safely and reliably, although we always have a recap on safety and technique each time. However, even with a reasonable iron, they take a lot longer than an moderately experienced adult would. They don’t yet have the instinct for which direction to approach each joint or how short to clip a lead. They get the solder wire stuck to the work and then panic, etc. I would not want to monitor more than two irons per adult at age 10. So far I have always done one-to-one. Meggy is also a bit unforgiving because none of the chips are socketed so you need to be careful not to bake them and if one goes in backwards it’ll be a real pain.
If you do a club, I would think you will want no less than two, preferably at least 3 adults: One helping them position a few components at a time, one monitoring a small number of solder stations and preferably one helping clip leads & check. If you have 12 kids and 2 irons, I would suggest you will require at least as long as it would take you to assemble all 12 kits sequentially.
If it was me, I think I would take a middle-way. You could pre-solder certain components, such as the ICs, to the boards, for example, and leave the kids to do the discrete components. That way you cut out a load of joints and the risk of frying the chips while still giving them the satisfaction of learning to solder and hopefully leaving some time to investigate programing their creations.
I’d be interested to know how it works out.