Overbooked flight

I checked in this morning well within the required check-in time at OR Tambo with my two minor cousins to fligh to Cape Town. Upon check in I was told that the flight is full and I must go on the next flight. I told staff that I couldn't fligh later with the girls. Upon my question of how it is possible that they just gave away my booked and paid flights to someone else, I was repeatedly told "It is SAA's policy. You have to take the next flight." We pre-booked our seats so that the girls sit next to me because the are 7 and 11 respectivly. Now we are on a next fligth, not seated together and cannot attend the family commitments due to the delay. Staff said there is no management they can refer me to and it wont help in any case. We gave away your tickets (with children) thats it. Deal with it. Disgusted, sad, embarressed.