Flight overbooking

I booked for my daughter and granddaughter to fly from Lanseria to Cape Town at 07 30 on Saturday morning 13 February in order for her to attend her grandmother's funeral. at 1100 and with her due to arrive at 0930 there was no issue with time. I received an email confirming her reservation and she arrived at the airport to check in where she was told that the flight was overbooked and she was bumped to the 0930 departure flight. She called me and I spoke to various people at Lanseria and the attitude was simply that they are allowed to overbook but would receive a voucher. The fact that she was travelling with a 14 month old baby to attend her grandmother's funeral counted for nothing. I had to drop her sister at the church, go back to the airport to collect her at 1140 and we got back to the church in time for tea. As far as I am concerned I had a contract with the airline for carriage at a specified time which they confirmed, the result being that my daughter and I missed a family event of major importance. I know they do this to cover no shows, but no shows forfeit their fare, and another paying passenger fills the seat so they get a double fare. Not fair. The vouchers mean nil