Combine rhubarb, strawberries, water, and sugar in a medium pot and bring to a boil. Reduce heat and simmer for about 20 minutes, or until fruit has broken up. Add red food coloring if you want. Strain the syrup into a bowl through a colander. Then strain again through a mesh sifter, this time into a jar or a tupperware that you can store in the fridge. Don't throw away the excess fruit. You can use it to top some waffles...or you can eat it with a spoon like I did.
Chill the syrup thoroughly, at least 2 or 3 hours.
To assemble sodas, in a glass combine 2 tablespoons of cream with 1/2 cup strawberry rhubarb syrup. Add ice, then top with 1/2 or 3/4 cup sparkling water, or to taste. (It's important to mix the syrup and cream before adding the soda, otherwise the cream will curdle.)