setup item in a PO receiving

I have scenario where Item A is the parent or main item and A1, A2, A3 etc are child Items (Item A comprises A1, A2, A3 etc) when you create PO OR receive item, you expect to Put only Item A and other Items A1, A2, A3 should be populated.. so instead of inputting 3 or 4 items, u want to input only one item but PO OR receipt has to be done for all please please can any person help me ,this is an emergency!