you are correct, there is no way to lock out Master Fader, but of course you can with My Fader as you already know.
in the app store in the upper left-hand corner is a selection of just iPad apps,
click on that,.... then choose iPhone,
then download My Fader 3 for iPhone and it will run on the iPad so you can set access limits and lock their iPads from accidentally messing with the wrong mix.
I highly suggest downloading and installing Master Fader 3.2.1.
Dan