The thing is though, it depends how you and your kid see the device. If it's coercive, sure they'll turn it off. If it's a device you both agree its worth using so that the kid can have a bit more freedom and 'it stops Dad worrying', then turning it off really isn't an issue.
I have 6 and 9 year old daughters and I live in East London. We're lucky enough to have a nice large open area close to the house, with woods and fields. I don't have a tracking device for my kids, but I do give them a walky talky. It means they can go climbing trees and mucking about on their bikes without me feeling I have to helicopter too much. I've *asked* them to just give me a buzz every half an hour or so just to say their OK and they're perfectly fine about it. They can roam further than I would normally feel comfortable letting them go solo, they get a feeling of independence and responsibility.
Win, win really.