I mentioned consent to cover 18+ "kids". What I said is that if they know about it it's always ok (because they're consenting, so there's definitely no issue.). If they're over 18 and they don't consent, I feel like that should be illegal, but I don't know for sure. If they're under 18, I'm pretty sure they don't need to consent, since parents can basically make them do whatever.
So we deliberately designed the system to be 2-way - both parents and kids need to consent and are aware that their location is being broadcast.
Additionally, on iPhone you can always tell when location services are running as there's an arrow that appears in the top bar. You can also see in Settings->General->Location Services which applications are using it.
Not certain on any of that, though.