The swear ‘bloody’ in British English is a contraction of the words, “By Our Lady”, which is swearing using the Mother of Jesus, or so I have read.
IIRC, that is one of the least likely reasons for the term.
My best guess, after checking several sources, is that it was naturally acquired in the war-torn British Empire. Much like "scream bloody murder" in the US, I am sure there were many a military man in the British Empire, and many a 'reporter' who who visit the scene of their latest conquest and say, "what a bloody f#ckin mess".
It is then possible that 'bloody' was used by the children, without the profanity, and became 'ingrained' into their culture.
The original question is still the use of the word 'bleeding' as an adjective, and what the author meant by it.
What, for instance, does it mean to you, if you use it?