Wow! How to do things different next year...?

My kids (5 y.o girl and 9 y.o stepson) come from a large extended, generous family. This Christmas, they made an absolute haul of wonderful gifts and general outpouring of love. Today I took them to the store with me for detergent, etc and all I hear was, "Gimme..." and "I want..." and "Buy me...." I felt sick to my stomach. All I could think was that I had two greedy little monsters on my hand. Once in the car, I told them that there were kids out there who didn't get ANYTHING for Christmas and all my kids could do was demand more and I couldn't believe how greedy and ungrateful they were. (They didn't respond) I don't want a repeat of this behavior next year. Any ideas on what to say or do??