I haven't tried it myself, but you could try substituting freshly brewed and cooled coffee for the milk that's called for. The cake will have less protein and may require adding a little bit of extra oil (around a teaspoon). If you do try it, please let me know in the comments! Alternatively, you could add 1 tsp of espresso powder to the batter and 1/4 tsp to the frosting. Just be mindful if serving to children and go for a decaffeinated espresso powder.