This article isn't very useful because it doesn't distinguish between liability and collision coverage. It doesn't matter how young the driver is, if you put them in a cheap small used car, you only need liability coverage which is pretty cheap.
If you have a child on your policy, and you have a collision on your policy, then your child will be required to be included in that policy. Insurance companies don't accept that you can live in a household, but won't be allowed to drive the new car, so they will charge you like the child is driving the new car under any circumstance.
I think it'd be reasonable aside from one obvious edge case: my adult child is blind and will never drive a car, yet I have to sign a new affidavit every time I renew my policy (every 6 months).