We all have iPhones. Built in is an app called Find My. It lets me see where my wife and daughters are, and has proven very useful when I need to come rescue them due to a dead battery, or pick them up in an unfamiliar part of town, or find them at the fair.
BUT...
All of them know how to turn it off, because I made a point of making sure they knew how, and some of them have occasionally done so. I don't mind this, because our relationships are built on trust. And if I didn't trust one of them, especially my wife, fixing that would become Job One.
So drop the safety excuse, and just promote its utility. Start by showing them how to track you, without insisting they allow you to track them. Show them how to turn it off. Lead, don't just command or demand.
On the other hand, if the app you want to install pries into messages, or email, or other such things, you have way deeper trust issues to resolve, and fixing that should be your focus.