One thing I worry about, as a parent in Silicon Valley, is that my kid will somehow procure a phone and hide it from us. My kids don't have enough cash to go buy a new phone without us noticing, but used phones are pretty cheap. Also, a wealthy friend/boyfriend could buy a phone and pay for cellular (MVNOs are quite inexpensive these days), which would defeat router-based monitoring. My kids are currently too young to do any of this, but I foresee it as an issue in the future.