I'm travelling with my 89 year old mother this summer; this is our 4th European trip together and I've learned a few things from past trips. Your parents are younger, and my mom is pretty mobile, but I have full responsibility for everything and basically, it's like travelling with a toddler.
Hotels: I have certain deal-breakers on my accommodations bookings:
1. I now book only hotels that have elevators, or Airbnbs on ground level
2. Rooms must have a walk in shower. European tubs are usually bigger and deeper, not worth the risk of her slipping and falling.
International flights: we pack as lightly as possible but I realize that I might become responsible for both bags at any time, so we check our baggage for all flights. We always fly into Frankfurt and I make sure to allow 3 hours before the connecting flight to make sure we can get thru immigration and navigate to the next terminal at a comfortable pace.
Inter European travel: Buses are a little easier than trains since the driver stores your bags under the bus, but we both love trains, so I usually just suck it up and realize I have to get her, then two suitcases and then myself with a backpack, up and down the train stairs. Trains with transfers or connections that are anything less than 30 minutes are verboten in my travel-planning world.
Book everything possible in advance: trains, buses, transfers, tours, museums.
Travel Medical insurance is a must, IMO. A few hundred dollars, although an OUCH, is worth the peace of mind. I have Blue Cross and she has Medicare supplemental, so medical costs outside the US are covered, but I also purchase medical travel coverage so the costs are covered up-front at the provider.
Travel Evacuation insurance is also a must. Some policies cover evacuation only to the nearest facility. I want evacuation back home to the US at our choice and to include a travel companion. You need a huge limit. There is no oversight or governance on air ambulance or medivac fees. Bills can easily, and usually do, exceed 100K.
Sorry for the elephant in the room, but that evacuation coverage needs to cover mortal remains.
Taxis, especially at the end of the day, are worth every penny, for senior travelers.
Plan everything, but be FLEXIBLE.