Wiktionary explains the origin of the term molly guard:
Originally a Plexiglas cover improvised for the Big Red Switch on an IBM 4341 mainframe after a programmer's toddler daughter (named Molly) tripped it twice in one day. Later generalised to covers over stop/reset switches on disk drives and networking equipment.
What specifically did this switch do? Which parts of the computer were powered down or disabled? Was there any direct danger to people (e.g. electrocution or moving parts), or did it merely protect the equipment from fire or other damage? Was there any particular reason to put it at toddler height?
(If available, a picture of this specific button and its guard would also be nice.)
This question discusses the general properties of Big Red Buttons, but does not address specifics to the IBM 4341, which is notable for causing the invention of the molly guard.