I've been asking similar questions myself as I had a week booked from 16th May, plus a short trip next week. It is hard to see how the itinerary you describe could be feasible by the date your parents travel.
A magnitude 7.9 earthquake leaves widespread damage over a large area, and the areas your parents propose to visit are in the worst-hit area. Many roads are impassable, and in Chitwan in particular, there's a good chance the hotel is not standing. Large areas are without running water and electricity, and sadly disease and increase crime tend to come in the wake of such disasters. You say it was booked through a tour operator. Contact them now, and ask about cancelling. If the operator is based in Nepal without a overseas agent, this may not be possible.
You don't state your parents nationality, but many governments are advising against travel to Nepal and are evacuating their citizens. If that applies to your parents, most travel insurance policies will pay for cancellation. This is the case even if the flights and tour were non-refundable, as it'll be the insurers rather than airlines and tour operators that pay your parents.
If this is part of a longer holiday, explore the possibility of booking something instead of the Nepal section of the trip. Most insurance policies will be happy to cover the cost of that, as it's typically cheaper to pay for a replacement week in the middle of the holiday than covering the entire cancellation costs. But do speak to your insurers before doing that. I've been pleasantly surprised by how helpful mine were in discussing options over the phone.
If your parents are uninsured, they need to think very carefully about what to do. Cancelling and accepting the loss may well be best. However if they're unwilling to do that, most airlines are still serving Kathmandu and most of the western-style hotels in Kathmandu are relatively undamaged. Probably the roads will be clear enough that it's possible to get a taxi around Kathmandu, and if not, there are hotels along the ring road by the airport perimeter. But quite probably many of Kathmandu's tourist attractions will be closed. They may be able to get an internal flight to a relatively unscathed part of the country such as the far east, and salvage something of a holiday there. But unless your parents are the sort of 60-year-olds who would be willing to risk sleeping on an airport floor, this is not a sensible option.