I believe the main reason why kids can't OLCI (and with PYOBP) is that theoretically, the child could OLCI and PYOBP and could travel by themselves, without the family. To prevent that happening, BA disabled OLCI if there are kids in the booking.

I think it's to do with liability issues. I don't know when/if OLCI 2.0 is coming, but I would hope that this would be one of the issues they would deal with, along with true multi-pax OLCI. I'm not holding my breath though.