All of my kids are signed in on my iCloud account, so I have experience with this issue. You can keep this from happening just by changing the settings on the phone. Go into General--FaceTime settings. You can select which phone #s you want to receive Facetime. Parent number is obviously selected. Uncheck it.
Anytime you do a software update, if the person installing it doesn't know to only select their number(and not the other numbers/email addresses on the account), you can have stuff like this happen. I have had it happen several times with my kids.
It may not be quite as creepy as it seems. Let's hope.