High level: Pretty much all modern technologies require energy at their foundation to work. Invite them to sleep on that first.

Secondly, for money in particular, over thousands of years of intense selection pressure, we've learned a few characteristics of what typically makes money. One of those characteristics is "unforgeable costliness" (https://nakamotoinstitute.org/library/shelling-out/). That is, for a thing to be a good money candidate, it must be costly to make, to the extent that it is extremely difficult to make forgeries of.

Then give the concrete example of gold. Ask them why they think vastly different cultures in different eras all converged on using gold as money? (Fiat was an anomaly). Given their age, they probably have a decent understanding of gold themselves and/or they've seen first hand how people flocked to gold in times of uncertainty, such as in wars, recessions, etc.

Answer: The energy to create gold atoms is unfathomably large (collision of neutron stars). It is impossible for humans to forge gold (even with today's technology). This is precisely why gold was able to become good money. Again, this probably will resonate with them given their age and their familiarity with gold.

Bitcoin actually "copies" this characteristic of gold. Bitcoin mining consumes energy in order to ensure that each bitcoin minted (and the Bitcoin ledger in general) is exceedingly difficult to make forgeries of. If you compare the energy required to synthesize gold, to the energy required to create and maintain Bitcoin, Bitcoin is actually orders of magnitude more efficient than gold. It also inherits all properties of digital things and be able to support (almost) instant, borderless transfers, something gold can't do.

So the general approach here is to give them an anchor (gold), then explain Bitcoin from that perspective.

(FWIW, I also wrote the below article a few years back on the Anatomy of Proof-of-Work. Perhaps a bit too technical for grandpas though.) https://medium.com/bitcoin-tech-talk/the-anatomy-of-proof-of-work-98c85b6f6667