Damned if you - damned if you don't.
" 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."
Interestingly, in this case, can the child be classified as "unaccompanied" if mom is there?? Also, if another passenger volunteers through "goodwill", can it be argued that that passenger has been asked?
Regardless, my position would be that the capt has to take into account the presenting information, his understanding of company policy and his esteemed professional judgement to make a decision. He is there - he is responsible for that a/c and I am sure that in any set of similar curveball circumstances different pilots will make different decisions for different reasons and be equally able to justify that decision.
Judgement - for the capt.