Due to their fault-forced to take a later fllight

I was booked on the 15h35 flight back to Capetown. When I wanted to book I was told I must check in only at 15h00..due to somerhing to do with fueling the plane. At 15h00 I went back to the counter where I and a few other passengers were told that they have \over-fueld the plane"and therefor we must go back on a later flight. The reason why I am so upset is I specially booked on an earlier flight back as my daughter is a disabled child that got her WP colors and a special awards evening was arranged tonight at their school. This is most properly a once in her life time. Now I miss an event like this due to a mistake on SA AILINK SIDE..Then they came and give us a FREE return voucher....but must still pay the airport taxes which is in any case more than half of a normal ticket...How must this now make up for this special event?????All I can say....BAD BAD BAD SERVICE..."""