I don't know how to do a DDoS or steal children's personal data from VTech either does that mean it can't be done
I am well aware of hacking, but the printer connects to HP to update on how many pages printed etc, so you're not directly hacking the printer.
As I said, you'd hack the PC (via the router) and then you could potentially see what's in the printer queue and maybe even access cached data.
You wouldn't find the printer and somehow see what was bring printed at that very moment in time.
With IoT, it's obvious that the media will be latching on to any stories about hacking. I mean, hacking cars, fridges, doorbells etc.
Many hacks are 'proof of concepts' which rely on so many things to be correct that ordinary people have little to fear, especially with most modern equipment not coming with a default admin/password setup. People have to set up new equipment and subscriptions with unique passwords.
Of course the weak link for online services is when that gets hacked, but I do think there are going to be many scare stories circulating in the coming years that are just that.
Anyway, back on topic, I don't think hacking is something people should worry too much about if considering going for the HP Instant Ink service. If there is a concern, maybe wrap the ink cartridges in tin foil first.