Forced to pay for a flight for my child

I booked a flight for my daughter and I to go to Durban. I amended this flight twice and eventually decided to make this trip in May. I did the changes online and paid the difference. On 31/05/2015 I was at King Shaka airport. The assistant at the check in counter said that I did not pay for my child. She referred me to the Mango helpdesk. I was confused as I followed correct procedure when amending the booking. Here the assistant notified me that on the initial booking my child was under the age of 2 and after the amendment date she was over the age of 2 and I would have to pay full price for the flight. I was told that I could board the plane but my child would be left behind. How does Mango expect me to fit this last minute bill of R624.28 seeing that i am a single mother? I really could not afford this last minute expense in my budget. I had to call family relatives to transfer money in my account. Absolutely disgusted with mango and their manager Samkelisiwe Mkhize who showed no sympathy. Their online system did not pick up that my child was now 2 and I was forced to pay this amount without notice or choice. Not the way to do business ! ******** people.