Even though you are buying the parent you need to have it set up as a "M" type part. If you have it set to P then MRP will stop at that level. Realy you are not "buying" the parent, you are using the supplier as a sub contractor to make this part from another part. So all the purchase orders for the parent should actually be sub-contract orders linked to a WO to make it. The lower level item would then be purchased and shipped directly to the other supplier. Once they receive it you could then do a wo-issue to the WO. When you get the finished item in you would then do a Sub Contract PO receipt to pay the vendor for the work and then do a WO receipt to put the item into inventory.