Until your main laptop is unavailable for some reason, so you borrow the kids one to do that urgent bank transfer or buy something online. Hopefully at that point you'll notice the amount of adware and crapware hogging the laptop and compromising it's performance that the kids have just got used to, and realise something is wrong before handing over your bank details to the Russian mob.
I think the point is that an adult mostly risks losing money from their bank account (assuming they do their banking online), credit card (if they buy anything online) or the like.
Having a compromised machine on your network serves as entrypoint. The machine may also be part of a botnet which could expose you to questioning or worse by the authorities if the botnet is used in an attack.
neither is something you'd want, even if the machine itself is not used for anything critical.
Edit: I'm not defending Dell in any way, but if they're watching youtube and browsing facebook, they'll probably be just fine.