Cleaning and removing roofing tar from skin completely can be a hard task. Roofing tar is really messy and hard to remove from your skin. In my experience I have run across two things that remove it from skin very well.
Cleaning removing roofing tar from your skin is a task soap and water just will not do. Most people do not have professional grade cleaner around the house to remove roofing tar. Most households do have mower gasoline or some small amount of gasoline around which will remove the roofing tar almost completely from your skin. It does require immediate washing of the skin with soap and water afterward. GAS IS FLAMMABLE so be careful and think about what you are doing. Please check for sparks or open flames first and use only outdoors. Lawn mower gas is a good source for this, and yes it will work if it is the weedeater gas oil mix.
You can also use a spot remover called "Goof Off " to remove roofing tar from you skin. This product is sold at Wal Mart, Lowe's and those kind of places. Washing you skin with this product will also remove roofing tar, again wash skin immediately with soap and water.
Both these methods require adult supervision and common sense. Please think about the environment and correct disposal of waste.
Sand is something you can also use to remove roofing tar from your skin. Just grab a handful and "wash" your skin with the dry sand the abrasive properties will help some to remove roofing tar.