Do you mean spending a night or two in California, or just a layover for a few hours?
Hawaiian and Delta do nonstop flights from JFK, and those would be my preferred options. They use larger, long haul planes, which are more comfortable than using two 737s or 757s IMO.
I wouldn't stop in San Francisco or LA just to stop, only if you had something specific the whole family wanted to do. You could do 2-3 nights in
Disneyland, but that would add substantial cost to the trip and might not be doable.
If the length of the flight is a concern for your parents, I might recommend doing around a 4-hour layover and eating lunch at the airport. That way they'll have the chance to walk around and be well-fed before the second leg. But if you do this, you'll almost definitely be in narrow-body planes for both legs, which goes back to doing the nonstop flight.