Such a nice gift idea! And you get yourself a cute pair of shorts aswell ;)
Just cut(or unstitch)the jeans from the middle seam, and unite the two parts of different shorts together! Take a close look on how it´s sewn next to the zipper, so you know how to sew the parts together afterwards.
You can do as in this model, and leave the waistband from only one colour, or cut it directly in half!