Credits are not separated for QS and for the Deluxe plan. TS credits are separated on the regular plan. Children under 10 on the regular plan must order from the children's menu at TS.
As for using QS or deluxe credits to order adult meals for children, some restaurants may allow it and some don't. It is for sure that the DDP brochures still say "children under 10 must order from the children's menu." It is likely you would not be stopped from ordering from either menu at counter service. At table service, on the deluxe plan, you might have success and you might not.