Imac - How stop ipad access to all users please?

Just bought the beauuuutiful 27" 3tb fusioned i7 Imac, and incredibly happy, of course 😉

But I notice if a phone or ipad is connected, and user can access it??! Surely there's some way you can assign the ipad/phone to a login to protect it's content?? I don't want my kid messing with my phone/pad if I leave it plugged in. Both phone and ipad are pin locked.

Just wondered! Many thanks in advance for any help at all 😉