In a small pan combine the rhubarb, sugar, and lemon juice. Then set the pan over medium heat until it beigns to simmer.
When it begins to simmer reduce the temperature to low and allow the mixture to cook, stirring occasionaly, until the rhubabr has completely broken down.
Use and immersion blender, or regular blender, to puree the rhubarb mixure. Then you can pass it through a food mill, or cheese cloth, to strain out any pieces. Let the puree cool for about 10 minutes.
In a medium bowl add the butter, cream cheese, and icing sugar. Use an electric hand mixer whip together until light, fluffy, and smooth. Add the cooled rhubarb puree and mix again until fully combined.
If your frosting is too thick you can whisk in some milk to achieve the desired consistency.