You can't use DL miles for premium economy. DL doesn't offer it (as a separate cabin; DL does have the extra pitch EC seats that come with an extra fee for nonelites rather than separate fare classes), so your choices would be either low coach (where you could pay the EC fees, but it's not a real premium economy experience) or low business class award tickets. Who knows what availability will be for award tickets to/from Wuhan, but it's reportedly been good on the A380 CZ routes. Of course, you could also route the parents on connections through PVG, CAN, or PEK using CZ or MU with DL miles, including the DL flights exPVG or exPEK if desired.