What would you do as punishment if your child did this?

I dished up some pizza for the kids for lunch and when i turned my back and my 6yo went to the toilet, someone took her pizza. My 10 year old step daughter said the baby (aged 3) did it and i told her off for pinching food. I had to get miss 6 another meal and it was a real inconvenience... so anyway fast forward an hr and my step daughter let slip it was in fact HER who ate it. I told her she is old enough to know better if she was still hungry she could have said and she just sulked like a big baby and refused to apologise for blaming it on Evie. So i felt sooo bad for going off at Evie for something she didnt do. How can i stop my step daughter from lying to my face?