I agree with immensed about the assistance that your parents can receive if they use wheelchair. That's really helpful. I don't know if someone already mentioned this here, but you have to tell your parents to be very very careful with the luggage in India. My parents also returned to Nepal via the same route, everything was perfect except that they didn't receive their luggage in KTM airport. Reason was the luggage did not come from Delhi.They did not load in the flight that my parents were flying.  It was stuck there for more than 2 weeks. After lots of form filling, claiming, arguments, fortunately luggage did arrive  but that was really pain in the neck and bags were torn out and some stuffs were missing.So please tell your parents to check  the luggage in Delhi after they land there and to make sure they are boarded for ktm flight.