Clara HD How to permanently disable the backlight? (alternate firmware? config file? hack?)

Context: I bought an e-reader precisely to not have backlight like with all screens we have at home (TV, computers, smartphones, iPad, etc.). This was a major argument for buying: if it's too dark, then like with a real paper book, you just turn the room's light on and that's it.

I know we can just turn the brightness to 0%, but is there a hack / another firmware that allows to turn it permanently off?

Or another version of Clara HD with no backlight at all?

Reason: I don't want my young children to always turn the backlight on (and they do even if i turn brightness to 0%): I precisely bought this device to avoid them staring at backlighted screens all the day. (If at the end it's the same result as giving them a backlighted iPad, it's useless to use an ereader!)