The stuff you get in the shops is pretty dilute, and hence fairly safe. Buy them a pair of cheap goggles while you're at it. Anything with sodium hydroxide in the ingredients is good. Sodium hypochlorite is a bonus. I'm a rare oddity in being both a plumber and a chemistry graduate!
Plungers cause all kind of problems, as the compression they generate can wreak havoc on pushfit and compression connections, such as the ones that connect the trap to the pipework. There's always a better solution.
If you want to give them a decent means of mechanically unblocking waste pipework, buy them a TITAN wetvac from Screwfix for around Ā£50. The vacuum in the pipework is unlikely to cause leaks. They'll need to cover the overflow of any sink or basin in order for it to be effective if they're sucking out from the waste AKA plughole
Benedict
Pedal Powered Plumbing