Tell your kids to stop screwing around with your manual speaker Channel Levels and Remote Control.
In fact, hide your remote and just use your iPhone as the MusicCast remote like I do. My physical remote doesn't even have any batteries in it - hidden in the closet.
I set mine to reflect ~ 80dB (about +5.0 above the standard Automatic Setting), so my "usual" level is always about -30.0.
-20.0 would usually be extremely loud on my Yamaha.