There are two issues
1) Their safety (running soldering irons, active 230v projects, high current batteries on charge)
2) The safety of your tools, since some tools are easy damaged,
* ultra fine wire cutters get used to cut thick cable (a problem with older kids and even some adults)
* antistatic tweezers get their tips bent
* LCD screens scratched with objects or things melted with the iron if they get hold of it
* Carbide PCB drill sets broken or tiny drills lost in the carpet.
As long as they have 100% supervision there isn't any issue but i wouldn't be working on a project at the same time. It's risky to try and keep an eye on them and work on something at the same time.