(Eeek...you DID say the child was holding a cookie in his/her hand! Norma, I found this one. And, it is soooo simple :-))

Use a paintbrush to paint patterns on baked cookies with
light corn syrup, then dip in colored sugar or sprinkles.

Corn syrup may be tinted with food coloring.

To color sugar, mix 1 drop of liquid food coloring into 1/2 cup of
sugar.