Royal icing is best for decorating cookies, since it dries to a plaster-hard finish. This recipe yields enough to cover about 100 three- to four-inch cookies. To ice all those in this story, you will need to make two batches of icing; increasing the recipe by half won't work well.
I make royal icing ALL the time to decorate cookies and I can tell you that when it doesn't have any added flavoring it tastes AWFUL!!!!! Add at LEAST 2 tsp of your choice of flavoring - I add 2 tbsp to mine when I make a batch with 2 lbs of sugar. It tastes heavenly! My favorite is butter/vanilla.