sounds like what your trying to accomplish is a way to have the xbox basically retail except for when you use it? that can be done through ndure using the UDDAE...
UDDAE - This is a backdoor rescue dash that is based off of the Easter Egg Exploit, the uberdash, and an audio exploit. The thing about UDDAE is it allows you to completely turn off your softmod, going to a retail, unmodded state. Now, once your softmod is deactivated, how are you going to activate it? In order to switch it back on, we need to change some files around on the Xbox, and we can't mess with the files in an unmodded state. Basically, the problem lies in we need to have it modded in order to 'turn on' the mod. Which really makes no sense. This is where UDDAE comes to the rescue. Even with the softmod deactivated, we have a way to run homebrew code through a set of actions in the MS Dashboard. While this is a great feature for advanced users, it will not be used by the average user, but it doesnt hurt anybody by installing it. If you are interested in this, you can read up on
SCEEE and
UDDAE HERE.
e/ shoot, didnt realise you wanted family to still be able to use xbmc... you coud always flash with a .ini bios, have multiple dashes, and just change the primary boot dash whenever you stop/start using the xbox, keeping seperate partition directories for the stuff you do not want family to have access to, and make sure the xbmc for family doesn't have those directorie(s) you wan hidden from them...