Drive Custom Property from Assy to Child Part

Is there an expression or api that would allow me to drive a custom property from my assy parent to a child part?   I guess it might be the other way around have the child assembly pull a custom property from an assy.   I'm trying to create a link that allows me to take a finished link and drive a raw material part and quantity based on an equation. See below for an example bill.

Lead (Finished) – Qty 1, UOM EA,    length custom property linked to dimension

                Raw Material  -  Qty Length + X, UOM In., (pull length property from parent and apply an equation)

                Scrap Material – Qty Length –X, UOM In., (pull length property from parent and apply an equation)

Thanks

Ryan