Our 4yr old has a tablet, which can double as a phone. Our 7yr old (6yrs at the time) had a phone, but just for the camera function (of course, that quickly changed to games, games, and more games).
I think the data charges should be lower. Regardless, I'm not going to argue about that (too much). If there's any government regulation, I think it should focus on having some kind of cap. End-user gets pre-approved for a certain credit limit, and they have every single right to have a LOWER limit put in place. If the user reaches that limit, BOOM, affected services are disabled.
So, in this scenario, I'd envision the parents initially having a $10,000 credit limit on their mobile plans. They see that they're approved for this $10,000, freak out, and have it immediately lowered to $1,000. Even in an emergency, $1,000 should be way more than enough, right? Then they get messages at 50%, 75%, and 90% usage. If somehow they ignore the fact that they owe $500, $750, and now $900, then services will be shut-off. If they are in fact ok with such a huge bill, they could call in and have the limit upped again.
And delayed billing...too bad. It needs to be near instantaneous, and if the user hits their $1,000 limit and the provider later gets notification that there's an additional $10,000 in charges...you snooze, you lose. I don't like the whole "ok, we can shut off service if it goes past $X,XXX, but we may not know about all the charges you've accrued, so you may in fact get a bill for $XX,XXX." No, it's a hard limit.
And there's quite a few pre-postpaid mobile plans, at least in Australia. So it's possible that someone would think they're getting a prepaid plan, but in fact it's treated as post-paid (in that you can go way negative in balance, and be liable for those charges).
For now, I just do (true) prepaid, and don't worry about excess charges. If one of my kids got hold of my phone and called China, the worst that'd happen is I lose about $10 or so of credit. No five-digit mobile bills in our future.