Please HELP!! Subitems to subtract from parent inventory

Hello. I am trying to re-set up our inventory in QB. I have a parent item (large) which has subitems (different sizes: 33', 36', etc). I am able to list these as subitems but is there a way when a 33 sells for it to subtract it from the parent totals? To clarify, I want the "parent" item to be our total inventory, say 100. and as the subitems sell to tally each size but subtract from the total/parent inventory. That way our inventory count is correct but we can also track size popularity. Hopefully that makes sense. Thanks in advance for any help!!