i am trying to lock the sound on my sons iPad

My son listens to his iPad with head phones. I would like to lock the volume in order to manage the noise level. I have tried general/sound/lock but this is not working. when he changes applications or hits the manual volume button it changes the volume and overrides the lock. help.

my son has a first generation iPad but we also have a apple macbook pro laptop and a iPad air so it would be great if you can help with this challenge for all three devices.

Thanks