@OP if you are really serious about teaching your elderly about Bitcoin you should have patience in guiding them. Don't use too many technical terms and as much as possible be simple. You need to keep your explanation simple so you have to find simple words that your elders will understand and replace those too techy terms. You can also give a graphical presentation, probably you can find a video on Youtube that explaing Bitcoin in simplest way.
Another thing, you should have prepared a lesson plan or guide so that you won't jump into a random topic, it should be organized. There are lots of tutorial guide online, then just simplified it according to the understanding of your grandparents.
You must be patient in answering questions if the elderly got interested, they will barrage you with a question so make sure you are ready for it. While teaching them, you must be hands on, meaning you must show them how it works by giving the example and walking them through the process.
Teaching the elderly is really hard, you can either have the patience or just give up and just mind your own business. I wonder why are you pushing your grandparent to understand Bitcoin. I think they are already comfortable with what they are doing and you force-feeding them with Bitcoin information may probably bring discomfort to them.
Elderly people they don't believe in technologies and the modern things in the world and it is very difficult to convince them to accept what it is. You don't need to worry yourself much if the people around you don't believe in bitcoin, just focus on yourself and do your thing. To me bitcoin is an individual race, it is left for one to accept it or not. Continue your race on crypto , sometimes mere words hardly convince people to accept things. Use your results in crypto investment to prove their ideas wrong.
They believe in technology and modern things, it is that they don't want to leave their comfort zone so they just ignore any tech since they just wanted to enjoy their free time.