"Aside from having parents block all third-party apps, which neither Apple nor developers want, the only option is to have a "your kid could potentially access adult content" rating."
They could parent protect the data connection. I think this would work fine for younger kids who don't need to be on twitter or whatever, and for whom parental protection is the most important (presumably).
With the arrival of the 3GS, a 1st generation iPhone just trickled down to my 2 year old. I just put it on airplane mode, but I am guessing that trick is only going to work for a couple of years.