I guess I am lost on this. If the child lives with the parent that is paying the child support are they paying them self for the child upkeep?
If the child has a job does not matter for child support, what matters is that the child is a full time student and under the age of 18, if this is true then the child will recieve child support until either he/she graduates from high school or is 18 years of age, whichever is later.

I am not sure what state you are in but I am in Michigan and child support continued until the child is 18 and graduates highschool. The child having a job does not matter - the parent still has financial obligations until then.