You can fly, but leave your 3 year old please

Flying from george to johannesburg, my wife and i were allowed to check in but as we did not have my 3 year old daughters birth certificate, we were told she would not be allowed to fly. I know this was a requirement for international travel but this is domestic, we flew down with saa without this requirement, but the attitude of simply, "Sorry she cannot fly" is ridiculous. Finding reasons not to allow people who have paid for tickets, to fly, leads me to not flying with these guys again.