Well, we've tried only 1 Diego kids DVD that was close by when we were fiddling around with the car. I wouldn't say intuitive at all. Takes some experimentation to figure out which buttons to click and also how to get the DVD to play without any iDrive buttons also being on the screen.
But, the volume difference is very noticeable. You should be able to show it to them without even using any words. I have no idea why it would be this way by design.
I haven't tried it with a DVD-A (DVD Audio) Disc because I don't own any. Maybe someone else has tried that.