Transit In London ??

15 hours is too long a period to be waiting in the airport unless they have access to Lounge where they can relax, eat something, and freshen themselves easily. I would recommend you getting a lounge access, if possible. Otherwise, a hotel inside the airport would be the next best option. If they are US citizens, there is no need for a visa, if they have to stay in a hotel outside the airport. If they have Indian passport, definitely, they need to have a visa. Then, they need to figure out transporting themselves to the hotel and back to the airport. If the travel is not in the near future, you can get a new credit card and give it to them with a specific low limit say $1,000. Even if they lose the credit card, you are not exposed to a big liability, unless you have fraud protection in your credit card. As there are two airports, make sure you check the airport they arrive and leave are one and the same. Some of the flights make take off or land from Gatewic while others may depart or land from Heathrow.

 

Instead of visa / credit card. Can't you call the airline and check options. may be they might have another route for some extra charges. anyway now you are introducing extra costs.

i do not know, this is just wild guess. can you check if the airport itself has a overnight lounge.

 

Are your parents US citizens?

 

I think the cleanest option appears to be getting them access to the lounges in Heathrow airport if they are in Healthrow for 15 hours. Every terminal has a lounge and the cost of becoming a member is 55 pounds and visit fee is 24 pounds. It gives them access to be seated comfortably with free meals and access to clean toilets. They will also notify them when the plane is about to depart so that they can go to the gate on a timely basis.