In a large kettle, boil the water and sugar until the sugar is dissolved.
Stir in frozen juices until melted. Let cool.
Pour into freezer-safe container with a lid. Freeze several hours until it's partially frozen (slushy). If it gets frozen solid, let thaw until it's slushy.
Pour slush into a large punch bowl and add the lemon-lime soda. (Equal parts slush and soda; if using all the slush, use the entire bottle of soda.) Or scoop slush into cups, filling partway, and pouring in an equal amount of soda. Serve immediately.
For an alcohol version, mix 2 cups vodka into the entire batch of slush. Serve immediately.