Unpopular opinion: Your kids do not actually have freedom if you're tracking them.
Even if it is "just for emergencies" and "we don't actually look at it".
I enjoyed a completely untracked childhood, and I will make damn sure my kids can have that too.
Just knowing that your parents trust you is a priceless feeling.

then yes you should have as much financial separation as possible.

Yeah that's a thing people in abusive households frequently have.

This is a problem even without this. The account owner can get lists of all outbound calls of their victim's line if they share a plan.

The fcc requires some remediation if a domestic abuse order is submitted but obviously that's at the far end of the abuse cycle.

The issue here can be traced all the way to phone companies pushing the very concept of family plans because it makes churn more difficult.

An abuser can shut off their victim's phone line on a whim with convenient online interfaces.

Phone companies don't treat their customers will respect because their is no requirement. No one of adult age should be subjected to any of these controls simply because someone else pays.

The health industry has rules around this. The moment a child hits 18, their claims disappear and the parent loses access to medical records.

There is absolutely no reason phones should not have the same restrictions but the industry lacks the will and will until the fcc or other three letter agency forces the issue.

The carrier can track a phone without sim card but it's not the case if you turn on airplane mode. The whole point of airplane mode is to prevent the phone from emitting any signal to avoid interference with critical aircraft instruments. I don't see any company risking to circumvent such a critical security feature, it would be easily verifiable.