Fire room has me in tears

Booking my son’s 21st birthday at Fire Room Mall of Africa has proven to be the most painful experience ever. Last minute they tell me of new terms and conditions that will cost me an additional 8000 rands that I don’t have I’ve booked a private room and paid for it and also a deposit for the food. If I could I would change the venue even though it’s tomorrow. They have mixed and different messages. I don’t know what to do or who to complain to. They basically say that because I’ve increased the number of people attending I have to choose a more expensive menu. This was supposed to be the best day for my son. Shouldn’t terms and conditions be explained before booking? Shouldn’t it be on the contract?