This seems like a very mushy reason. I'm pretty sure the real reason is that children are not considered able to knowingly consent to many things - including contracts such as EULAs or TOS. Given this, a child is also not expected to be able make a reasoned decision about privacy tradeoffs.
The reason it's "ok" to spy on adults is that they can make an educated decision about whether they're ok with being spied on. I don't necessarily agree that this is true in practice, but I think that's the theory.
It should also be completely illegal, but the justice system can't keep up with technology. Imagine you found out that your next door neighbor has drilled a hole through the wall and fed a camera into your house - what do you do? (Call the police, certainly). Is it really any different when the camera feed isn't a physical wire but done over the internet?
In reality the same arguments apply to adults, but we find it less morally repugnant because adults aren't innocent and are expected to gaurd themselves against such actions. However, it becomes more and more difficult to guard against.