Put all ingredients in a blender and blend for 30 seconds or until smooth.
Use a funnel to pour the watermelon juice in to the freezer bags.
Lay flat on a cookie sheet and freeze for 24 hours before serving.
Notes
If you can't get the freezer bags, this recipe would also work well to make adult popsicles on a stick. Pour your blended mixture into an ice pop mold and add popsicle sticks before popping them in the freezer. The freezing time for this variation on alcoholic popsicles will be similar to the freezie form.
If you are creating a variation with different fruit flavors, the only detail you need to pay attention to is the ratio of alcohol to juice. Because alcohol has a lower freezing point than regular ingredients, make sure that your ratio follows the rule of thumb that for every 4 ounces of juice, you want 1 ounce of liquor.
The easiest way to pour the liquid into the freezer bags is to sit the bag in a tall cup which will free both hands for holding the funnel and pouring.
Because alcohol has a lower freezing point, you need extra time for the ice pops to freeze than if you were making regular popsicles.