Kids are pretty damn smart when it comes to lying, unfortunately.
I think they are going to learn to come home, leave their phone there (or at a friend's house) and then go do whatever they want, secure in knowing they are definitely fooling mom/dad.
So you are right back to where you were before they had a phone at all.
I think you underestimate how addicted kids are to their smartphones. There's no way they're going to hang out with friends without it. How would they take their hourly facebook portrait?
I think they are going to learn to come home, leave their phone there (or at a friend's house) and then go do whatever they want, secure in knowing they are definitely fooling mom/dad.
So you are right back to where you were before they had a phone at all.