I was confused by your question. Are you asking if you can just purchase 1 Adult QS dining plan and 1 Child QS dining plan on a reservation for 4? Or are you asking if you can just use 1 QS credit at a time?
If you are asking if you can purchase fewer plans, the answer is no. You must purchase a plan for everyone in your party.
If you are asking if you can spend your credits 1 at a time, that is answer is yes.
Further, the QS credits are not broken out into distinct adult vs. child. So if you want to share 2 adult meals the whole way through you can.