You can get them an I phone if they are old enough, and install I phone tracker/find my I phone.
I'm against it as I think me kids are to young to have a phone, however the minister for war and finance reckons the below
1) they will always have there I phone with them and never leave it anywhere
2)they won't accidentally "lose it or take it off"
3) a watch as described above will be taken off if they are up to no good
4) if they do get into a scenario they can ring you and dad's there
5) if they don't ring you and get into a scenario dad will be there whether they like it or not........
6) as a contradiction of previous statements I don't want them to develop "trust issues"
7) it's hard to be a prison guard and a parent
8) it's not that I don't trust the kids I don't trust other people/people's values/morals/that there are no sickos
To summarize it's all a tricky slope to slide down and ya damned if you do/or don't
The world has changed since I was a kid and social values and ettiquette aren't as important to other parents as they may be to you......
Personally it is really age dependent and depends on the relationship that you have with your kids.
I was a good kid but still a crafty sneaky little turd at other times there's no way I'd wear that but I would of taken a phone said thanks in a heartbeat.
Whether your transparent their being tracked with them or not is a whole other kettle of fish.......