If you're wired (ethernet): ARP poisoning to get your traffic to route through the attacker, then listening there. Works beautifully on most if not all LANs.

Though even if it didn't, WiFi by itself would be concern enough: most if not all the internet-equipped private homes have (often badly secured) WiFi, and all public spaces (airports, schools & universities, restaurants & bars, etc.) usually use key-less captive portal WiFi APs where listening in is even easier.

So yes: traffic interception is a very legitimate worry IMHO. And as someone said below, beyond those concerns, you could also be sniffed by techs working at your ISP (and its peering partners) / Law Enforcement / etc.

Given that a child's laptop will likely be connected to the school's wifi, where any other child would then be able to sniff the keypresses...

Yes, it's a problem.

Not if you can trust everyone employed at your ISP and every company they peer with.