If it is from HKD, then in any hole in the wall place (they're everywhere) it is 1:1 WITHOUT a receipt. You throw a bunch in, they throw a bunch out. It's a 10 second process. If you request any documentation they either refuse service or jack the rates way up.

If it's from USD, then any place should be about the same except Travelex at HKG which is extremely expensive and at hotels.

How old is your son? If he's below HS age I would probably just get the RMB in the US. You'll only be saving a few bucks anyway (at the most).