I've seen these cookie moon phases before (click here for a description of the activity on Science Bob's Blog) and wanted to share my "Moon Phase" cookie story.
After seeing these online I thought it would be fun to do it in class as part of our Astronomy unit. I decided to make these at home. They turned out adorable. Then I decided to eat them (justifying that I would let my kids eat them in class :) It quickly became apparent that 8 Oreo cookies was way too many to eat (I definitely felt queasy). I went back to the online directions and found out that I was suppose to use "mini Oreo cookies" (which made much more sense).
A note of caution, the mini Oreo cookies may not be as cost effective with large groups of children (when I taught middle school I had 80+ children). It is definitely cheaper to buy the generic chocolate sandwich cookies. I would just provide a snack or sandwich baggie so the kids could take the leftovers home.