From their FAQ, looks like that would need you to deregister and reregister the phone. Still, as in my experience most parents (mine and others I'm familiar with) leave their phones lying around without a password so much of the time (or else likely use weak ones such as their date of birth or similar), that isn't exactly hard.

I've also accidentally shoulder surfed dozens of random people's (patterns on android are particularly easy as by default it shows the full trace; ios7 is also easy as the buttons are very high contrast) where I had zero intention to just because they aren't careful about who can see the screen when they unlock.

Of course, given the poor security of so many mobile apps, I wouldn't be surprised if there's a way to reverse engineer the protocol to lock a 'parent' phone. Then, of course, the child can feign ignorance with "but I thought that wasn't possible".