I actually think the idea of having a pin on the removal and adding of devices is a good idea, espeically where you have kids but also it adds some extra precuations to the remaval/changes process. Maybe even reduce the amount of people contacting live chat for 'Accidentally removing things'.
You could just use the parental pin for this as it would basically be a kind a prenetal lock. Though i guess inplimenting it would not be as easy as it sounds since all apps across all devices/platforms would need to be updated to support it.
Many other online services do have features like this though. I can't sign into or remove a device/app from my microsoft account without an authenticator code, google has a simular system, even the Yourtube app on the NowTV box requires a one time code to login to the account and add the device to your account.
