Why is it ethical to track children, but not adults? If phones existed when I was a kid, I would have been horrified to discover my parents had installed spyware on my phone.
However, ethically speaking, we should strive to track as little as possible, since the whole point is to allow the child to develop into an independent, self-responsible adult.
What's deemed acceptable to maintain control over a child has been diminishing (e.g. corporal punishment). Maybe location tracking should be the next addition to a child's Bill of Rights (and then we can talk to our Big Brother babysitter about knocking it off too).
(This, of course, in addition to regular parental anxiety about the safety of your kids)
In general though, well a natural or man made disaster can happen suddenly, and you probably want to know where your kid is then, especially if you are fleeing for your life.
Also, kids get kidnapped. Some kids run away. Also there are bad people in the internet, horrible people that find kids attractive and then groom children or teens.
Having a kid, and I don't myself have one but my sister does, seems to involve a lot of worrying about bad shit happening. Kids don't always make the best decisions on their own. Neither do teens; and if someone is late for curfew, checking the app when they don't answer there phone is better than frantically calling hospitals.