Wrong Order cant be exchanged for correct one

I bought 3 Pcs & Chips without chillis for my child and they gave my child meat with chillies. When we drove away my child took a bite she find that it was wrong order with chillies and we drove back. When we got we told the manager on duty about their mistake. She said 1 pc which was bitten they cant take it back, they can only exchange 2 pcs which where nothing bitten. I asked how was my child was going to feel that it was wrong order had chillies and what are going to do with there wrong bitten pc she said through it away but we are only exchanging 2 clean pc

Then i told her to take back that 3 wrong order , i paid for another correct one without chillies and left

Please help