If the main reason seems to be that you want to know where they are when they've decided not to answer the phone... maybe that's fine that you don't know. If you're expecting them for dinner, and they decide to do something else instead, they should learn that consequence: if they don't come home when they're expected for dinner, they don't get dinner with the family.
I don't have kids, so of course my opinion is irrelevant. But I do remember decently well what it was like to be a teenager in the 90s. I was a "good kid" and didn't get up to much that my parents didn't want me to do, but a) I did do some things my parents didn't want me to do, things that they would figure out real-time if they'd been able to track me, and b) despite me doing things they didn't want me to do, everything turned out fine. The idea that I wouldn't have been able to do those things, and the feeling of being trapped and constantly surveilled... that's gross.
And... you mention spouse, too? I would never let my spouse track me 24/7, and would never ask her or expect her to allow me to track her. To me, that would be creepy and an invasion of privacy. I get that some people do this (and know some of them), but I just think it's weird.