My son has had an old iPhone4 for a couple of years now, just to play games and take photos. It has never had a SIM card while he's had it so has never been used as a phone.
I'd like to put a SIM card it in but only if I can find a way to restrict its use so that he can't get online at all and can only use it to text or call a few key emergency contacts (as well as still playing the games/music and using the camera etc).
He has started to want a bit more independence and I'd feel better letting him walk to his friends and play around the village without me if he has a phone on him.
Set it up so camera, wifi and web browser can only be accessed via a PIN that only you know. Without going into settings and knowing the PIN, you can’t see an icon for them. That’s what we’ve done for our 9yo. You can do it all through settings.
It's a pretty good price for us, as we have 2 kids. What we've also realised is that after about 6 months, they're resigned to iPad turning off at 9pm, and find other things to do. I reckon we could just cancel the subscription, they'd never notice. If he knows iPhone's boring and only for calls, I reckon DS will just stop being tempted by it.