Yep, £50k sounds like a fair cut off (it's always going to be an arbitrary decision for the exact cut off)
If I was cameron I'd make a potential smart move and for the first year, make the cut optional. So, say to those over £50k: "we're stopping paying this automatically to you but if you really feel you need the extra £50 a month (or whatever it is) then you can apply to get it reinstated.
I would be surprised if many on over £50k would go to the trouble to claim it back.
I would also limit child tax credits to a maximum of two children per famil (for all citizens, not just those over £50k)