This is my son, Micro-Bash. He is one year and several months old. And he can unlock an iPhone.
For the past couple of days whenever he finds Mrs Bashcraft's iPhone, he switches it on and slides the slider, unlocking the phone. Nobody told him how to do this, he just knows. Kinda like how he knows to lock the front door and where we hide the cookies.
Perhaps, your kid can do this as well! Maybe your cat can! Who knows. You do, that's who.
Granted, he does not know what to do with the phone once he unlocks it. Cut him some slack, he's a little kid!
Simply one button interfaces mean that tech users are, for better or worse, getting younger and younger. Can't go much younger than one year old.