[tgjsrkghruksd]

Pure liquid mercury is not especially toxic so you're probably fine. That said it is damn stupid for kids to play around with it.

Lead is probably worse. I did a lot of soldering as a kid (like, from age 7 or so) and my ventilation/hygiene practices around it were far from ideal. I would not allow my kid to have that kind of exposure. If he needs to solder, then he needs to get a bench with proper ventilation and be scrupulous about clean-up and hygiene. Leadless solder is another thing but it has its own problems. Probably better is to do any electronic stuff in a format that doesn't involve soldering at least until more mature. These days it is not so important, though I expect it to be a useful tool indefinitely. All about proper balance.