If you are not going to be close enough to stop by the hotel and checkin for your parents, just do as Keeton suggests. Just give Marriott Rewards a call and order a paper certificate with their name on it. Unfortunately, Marriott's system can't handle ordering a cert in someone else's name and still have an e-cert. It has to be a paper cert.

I haven't used Pinniped's suggestion, but it might work as well. Though technically they could have some problems if the e-cert is in your account and you don't show up.

Also, if you are worried about them receiving and bringing the cert, have Marriott mail it directly to the hotel. I have done that a couple of times in the past.