It can be taken too far, but generally, if you paid for their device and you're legally responsible for their well-being and things they do on the internet, then I don't see the issue.

Combine this with the fact that the kid doesn't have to use the device for anything/everything.

It's like wanting to know where the kid is driving if you paid for the car, the insurance, and you're legally and financially responsible if they hit someone/something.

Again, I do believe this can be taken too far (and I don't know exactly where the line is). Paying for a phone doesn't give you right to spy on absolutely everything. I haven't seen anything that is beyond just app usage time, though.