A quick and simple way to control skin color tinting in VSS is to use the Color_Tint node located to the extreme left side of the material room.  In the render above, I used one VSSProp, changed the Shader rule to target each particular child, then used my default  and modified settings to each.  Again, I used only one VSSProp and the same skin texture for each.

As you can see, one child has gotten into some really bad candy, while the other appears quite normal.  The insets in the image show the modifications.  Notice, green boy still has some reddish tinge to his skin, indicates he is human after all.  If I wanted to modify that tinge, then I would alter the SSS node (unseen) color to give him an unearthly appearance.  But I am getting ahead of myself. 

I going to flag this for nudity, just in case someone takes objection.