Mix together 4 c. of water, lemon juice from lemons and sugar. You may choose to boil the water and sugar first so it dissolves, some prefer this so it doesn't separate quite as much. We usually don't bother.
Add watermelon, berries, and 2.5 c. water into a blender and smooth.
Add watermelon mix to lemonade mixture and serve cold with ice. Could add a bit of something "for adults" too.