How to explain money to kids??

How do you deny your kids gifts or certain items gracefully that you can't afford? Do you start to express that things cost money and we won't be spending money on that item?? 
We were setting up our Christmas tree and my son asked why there was no skirt on the tree and said we should go buy one. He didn't like the star and top and wanted us to get a new one. He wants his own tree for his room and extra items around the house. I wanted to just break down and shouted "we can't afford all that!"  But I don't know if it is right to talk to my child about my financial situation. 
Ugh the stress of the holidays.