I think the boingboing story misrepresents things, based on other versions of this story I've read - as far as I can tell, everything currently offered by BA is still free, but now they will also allow people to pay to get seats when booking the seats (at the moment that's not possible; I fly with BA a lot and I can only book my seats 24 hours ahead of time).
Also, as pointed out in the comments to that thread, there's nothing in the BA announcement about children - that was added by boingboing. In the UK, airlines are required to do their best to seat children near a guardian. So presumably, that will remain. What you will have to pay for is if you want to be seated next to your adult friend/wife/SO.