The problem is that devices have widely different output levels. Some mobile devices put out less than 1Vrms maximum, but headphone amps can output as high as 3Vrms. If you scale the headphone sensitivity to the mobile device, the headphone amp will play really goddamn loud, but if you scale them to the headphone amp, you will barely be able to hear anything from the mobile device.
What should ideally be brought to market, are headphones with limiters/compressors built in, so the maximum sound level delivered through them is ~85dB. The tech already exists for the professional market for hearing protection that feeds in outside sound through front-mounted mics, and caps the maximum volume level. But it will make consumer headphones more expensive, which is probably why no one is making them.
You could make a device that plugs in between the source and the headphones, with a calibration to the specific source and headphones, which can be set and locked by an adult. But that takes effort, and no one will probably bother to do it right.