Of course, we don't REALLY know what happened, so we can only speculate.
I would speculate that even the dimmest customer facing employee or agent of BA would hesitate to knowingly bump two children, but Taildragger (above) paints a plausible scenario that the girls were just picked off a passenger manifest with no details of their age or travelling companions.
Why is this considered to be an example of bad customer service. As has been pointed out several times, the girls DID subsequently fly. Is there any evidence or reason to believe that once it had been established they were minors that there was ever a problem accomodating them??? Again, I can only speculate, but it strikes me that whatever the reasons and circumstances for initially bumping the girls, someone at BA intervened and made the right call.
I've no reason to leap to BA's defence, but it strikes me that some people are determined to stick the boot in first and not worry about the facts until later.