If you were living out in the wilderness then I'm sure it would be less stressful but living in a built up area, especially the likes of London the risks are much much greater.
If you haven't seen it watch kidulthood to see just how harsh things can be growing up in London as a teenager and then think that those kids just get younger and younger.
Getting a mobile that's cheap and nasty might be worse than gettting them one that's just normal because kids still attach cheap and nasty with you for the whole of your life.
My daughter hid her self in the bathroom the other day and we spent ages looking for her
and when we did there was the emotion of either hugging her or giving her a smacked bottom but she was innocently playing in her own little world, so just imagine if she came home 2 minutes late from school or her friends house when she's older? We'd be calling the Feds and sending out search parties for her.
You may call me paranoid, but I say better to be safe than sorry.