I feel like there are multiple problems in your situation that need to be fixed. If you cannot contact your wife in an emergency there is a problem (setting off the alarm might not be viable in an emergency). Why not give your son either his own phone, a tablet, or an old phone of yours? Or why would your wife allow the volume to be off when your son is using her phone? If your wife is not at the location of the device when you "drop-in" this still won't fix your problem.