Enterprise Rent-A-Car has its only Walt Disney World location at the B Resort (former Hotel Royal Plaza) on Hotel Plaza Blvd. in the Downtown Disney area.
There are no car rental offices at any Disney-operated hotel at Walt Disney Word. Not at Pop Century. Not even at the Grand Floridian.
That being said, it should really not a problem for your in-laws to drive to Pop Century using any brand of car rental with a good one-way rate, unpack the car at the room, and for one of them to return the car at a local office and get a ride back to Pop Century. It's just important to pick a car rental office that provides rides to WDW resorts and to be aware of the office's business hours.
The car rental office does not have to be on WDW property. There some on-site (such as National/
Alamo and CCC and several car rental brands on Hotel Plaza Blvd.), and others nearby (such as Hertz on 192 and Hertz at the Gaylord Palms).
There's a useful sticky thread at the top of the DISboards Transportation forum:
Sticky: Car rental locations for WDW...