Simply put, I have a teen with an Iphone and he gets home before I do each day. I want to add his device to the automatic setting of “I’m Back!” routine, so he doesn’t trigger the alarm. That’s easy enough, but doing so requires installing the mobile app on his phone, thus giving him full access to the cameras among other things. I want his device listed on the app but no access to the app.
How can I accomplish this?
You’d have to use another service as a presence trigger, for example IFTTT or Life360.
@geko is correct. You’d need an outside source to run this. Life360 is where most people will turn.
A presence fob from ST will work but that costs $30.
IFTTT will work, but there are delays sometimes.
If you have a compatible router, ASUSWRT will work: [RELEASE] ASUSWRT Wifi Presence or DD-WRT Presence Detection using DD-WRT Router Script
Life360 is going to the most reliable option, and the one that doesn’t require any new hardware.
Here are the official instructions for adding life 360 as a presence indicator to SmartThings. You first install the regular life360 app on your teenager’s phone. Then you follow these instructions on your own phone to add life 360 as a presence indicator for SmartThings. Then you’ll be Able to use your teenager’s presence in SmartThings rules without having to install the SmartThings app on his phone. 
The consensus is in, thanks a lot all.