Paid for nothing

I just want to raise my concern, we as family had, when we flew to Durban on the 16th June 2019. The issue is regarding the pre-booking we did, and paid for, but at the time of checking in my wife had to argue with the person checking us in.The seats we booked were given to other people, my wife explained that in our group we have small children that is flying for the first time and that they cannot sit on their own hence the reason for pre-booking. We ended up sitting all over the plain and for me this is unacceptable. Why give us the option let us pay and then we don`t get what we paid for.