This is one of those things about travel involving your own small children. You have to take the responsibility yourself, not send the child on an intercontinental journey involving a third-country transfer with a "companion" who you state was known as not particularly travel-savvy, and whose abilities run to having their mobile phone go flat (they all last for days nowadays) at the first stopover, in midwinter, is something that probably looked convenient to you at the time, but is not a parentally-wise thing to do at all. And it's no concern of the airline, whose skills are in transportation, and indeed in complying with the law.
Next time please take the 8-year old yourself.