I'll chime in and I'm sure others will add comments, too.
First, don't expect them to be in the parks from opening to closing. If they take an afternoon nap at home then do the same at Disney World. Think about having an off day away from the parks to just relax. Alternate your activities with doing a ride and then a show which will provide rest. Even with doing a
scooter you still make an effort getting on and off the rides.
Practice with scootering at home by using the ones in the grocery stores and such. The grocery stores ones are harder, but it will be good to get familiar with one before leaving for Disney. The best practice would be to go to a local zoo and use a scooter there for a day. They will get a feel for crowds that way, too.
You can rent a scooter at the park or offsite. Offsite will be cheaper and you will have the benefit of using the scooter at the resort, too. If you will be using Disney bus transportation that will probably be the most challenging. Getting the scooter into position on the bus is not the easiest. You may even have to do this for them. Keep in mind there are normally only two spots per bus for handicap vehicles which includes wheelchairs and scooters. If you want to stay together you may have to wait for another bus if someone is before you. If you are using a car/van then you have to have enough room to fit two scooters.
When getting a scooter from an outside vendor you are responsible for charging the scooter every day. You have to fit two scooters in your room, too. Some get the table removed from the room to fit two scooters. This is another situation where you may have to drive the scooter in and out of the room for them. At least, you need to hold the door open if they drive the scooters in and out.
If you rent from an offsite vendor, keep the battery charger with you in the park. You may have to do an emergency charge up in the park if you run out of juice. Keep in mind that you need to get the appropriate size scooter based on their weights. The smaller scooters max out at 175 pounds.
Okay, now for the fun part. I was surprised, but my mom, who was 89 at the time, enjoyed meet and greets. She enjoyed Toy Story Mania in Hollywood Studios and Soarin' in Epcot's Land Pavilion the best when it came to rides. The great Movie ride in Hollywood Studios was another good one. The Safari ride in Animal Kingdom was okay, but there are bumpy spots as you ride which are a little jerky. The Buzz Lightyear ride was good in Magic Kingdom, too. We stayed away from all roller coasters. We spent the most time in Epcot's World Showcase where you can see live street entertainment and movies and do a few boat rides. The parades and fireworks were a must see.
I would splurge for the Fantasmic Dinner package; so, you can get reserved seating at the show. My mom really enjoyed the Cinderella Royal dinner in the castle. Some character meals were fun like the Crystal Palace in Magic Kingdom and Akerhus in Norway at Epcot.
Other than the dining at our resort we stayed away from going to many resorts for dining. It takes more time to go a resort for eats and it's tiring, too. We did go to the Animal Kingdom Lodge for a dinner on the day we went to the Animal Kingdom park. We did Boma's, but there are other choices there, too. We took a tour of the grounds while we were there.