Mango airlines call it a admin fee

My husband booked a flight on the 22nd June 2015 for our girls flying from Lanseria airport to Cape Town. We paid via credit card. Seconds after payment went through, mango airlines sent a confirmation mail. Immediately we saw that he made it for the 23rd June 2015 instead of the 22nd June 2015. Why this is so significant is because they are flying with him to Cape Town (his ticket was already booked and paid for weeks ago), and we decided on the spur of the moment to include the girls. The one is fifteen and the other one is 9. Before I made the booking I phoned the airline to ask them how do I go about making the booking concerning the girls and that they will be accompanied by their dad. They told me what I should do. After the mail came through ( which had the wrong date), we were immediately on the phone with a call centre consultant. Our little mistake of a few minutes earlier ( when payment was made) cost us R877 extra. They call it an ADMIN FEE. This is just ****************!!Surely there must be a grace period in which you are allowed to fix silly mistakes like these. This is just upsetting. I hope that no other customer makes a SILLY mistake like I did. #Disappointed