Hi Ashley, for the 3 day itinerary, here’s what I’d recommend for dining:
Day 1: Lunch and dinner at Old Faithful Inn or Old Faithful Lodge (several restaurants/delis in the Old Faithful lodging area)
Day 2: Lunch at Canyon Village and dinner at Lake Village or Grant Village
Day 3: Lunch in Mammoth, Pack or pick up to-go for dinner in Lamar Valley

Here’s more info on Yellowstone dining.

All the trails on the 3-day itinerary are fairly easy, so depending on your parents activity level, they may be okay. The most difficult is Grand Prismatic Overlook (~2 miles). If that is too much for them, they could do the Grand Prismatic Spring boardwalk instead. Upper Geyser Basin on Day 1 is long, but flat and mostly paved. They could do as much or as little of that trail as they’d like. In Norris Geyser Basin on Day 3, you can just stick to the Porcelain Basin loop to shorten the distance.