HELP!

So, I am making cookies friday with my mom in town. We are mostly doing this for Aidan. I am not actually making the cookies... Pillsbury made the dough and I'm slicing it and baking it. Anyways, the point of this cookie experience was really for Aidan to put sprinkles on cookies. I bought some cake frosting but my mom has informed me that won't harden on the cookies. So, I am in need of a cookie frosting recipe (not a glaze but actual frosting) that will dry! I found some online but they all had egg whites or milk in them and I can't imagine those keeping well at room temp. Ugh.

Signed,
Unexperienced Baker