I have done it with Guides, and occasionally with older Brownies, but when it came time to use the pyrography irons themselves I had a 1 adult to 2 pyrography irons ratio, with strict rules that if the adult was called away the irons had to be switched off and unplugged, as they do get very hot.
How long it takes depends on how big an item each person is making, and on the style of iron - electric plug-in pyrography irons hold their heat consistently, whereas some of the soldering irons only heat while the button is being pressed, meaning they cool down rapidly between each turn and take time to heat up again (and the girls can find it awkward to hold down the 'on' button whilst working). If it's something small like an eggcup where they have already pencilled on their name and are just burning it in, it would only take a few minutes per person.
I have done this at a district event with girls aged 5-15. We had 3 pyrography tools for about 6 Guides who were rotating between activities together, with 1 adult to supervise.
They were doing pegs, but I've also seen small discs of wood too. Pegs pegged to the table firmly means you don't need to hold them study & keeps other hand out of the 'danger zone'.
Designs were limited to spots / stripes /zigzags and initials (or short names).
Safety wise, as long as they aren't going to stab someone else with the tool, there is only so much damage they can do to themselves, and if they touch the hot bit, they are going to drop it pretty darn quick. They all know to be careful. We kept a bucket of water under /next to the table to quickly dunk any burns, and didn't even end up with anything that required more then that (not even proper, running water!).