If you accept my money then honour your deal

I understand that airlines sometimes experience delays and that this can lead to inconvenience such as my three youngsters having to re-organise transport from Johannesburg to Pretoria and travel in the dark. However, since the airline is prepared to accept my money to pre-book specific seats I feel that their failure to honour this should place the onus on them to refund me this money. About a month in advance I booked said youngsters to sit together at the front of the plane at a charge of R55 each. This was not honoured and they were reallocated random seats. Since this is not the first time I have experienced this I decided this time to request a refund of my money that bought me nothing. No joy thus far.