I made these cupcakes just for fun. :) Here are the simple instructions:
1. Buy a box of white cake mix – and make according to instructions. Before baking, tint the batter pink using food coloring. Add mini-chocolate chips and stir. Bake according to box instructions.
2. Frost with green-tinted buttercream. See recipe below.
The Repressed Pastry Chef: Buttercream Icing
Recipe source: therepressedpastrychef.blogspot.com 
1 stick salted butter – room temperature
1 stick unsalted butter – room temperature
1 cup shortening
1 tablespoon clear vanilla extract
2 pounds confectioner’s sugar (powdered sugar, 10x)
4-6 tablespoons very cold milk**
1. Cream the butter and shortening in the bowl of an electric or stand mixer.
2. Add the clear vanilla extract and combine well.
3. Begin adding in the sugar and mixing thoroughly after each addition.
4. After all of the sugar has been added and mixed thoroughly, begin adding the very cold milkâ€¦ one tablespoon at a time, combining very well after each addition (mixer on medium-high to high speed) until you reach the desired consistency.
**If you want to add a watermelon flavor to these cupcakes, you can substitute watermelon juice for the milk.