key fob or some RFID is much easier to use than getting out your phone, flipping through the apps, finding your door app, opening your door app, pressing the button (not registering), pressing the button (fuck, not registering), ahh there we go.. your door is unlocked.
If you're that worried about your nanny having a key, maybe you should stop trusting that person. A door lock only keeps out the honest anyway. Unless you live in a bunker.