If the rule has been correctly quoted as:
"easyJet does not operate an indemnity policy and therefore under no circumstance, under the Child Protection Act, must another passenger be asked to accept responsibility during flight of an unaccompanied minor."
I'm with all those who have pointed that the crew misapplied it, in what was certainly a genuine misunderstanding.
What the rule is intended to achieve , quite rightly, is that no ground staff member, or aircrew, should ask a passenger to look after an unaccompanied minor during a flight, for many good reasons including the danger of sexual abuse. We all know what an unaccompanied minor is; a young child travelling without parent or guardian, known to the trade as an UM, labelled as such and positively handed over at each stage of its jpourney until returned to a named parent or guardian.
The rule has little or nothing to do with the case on Easyjet, and had the Captain known this I am confident he/she would have acted with common-sense..