I'd tell them that bitcoin was humanity's *first* authoritative decentralized record system, and that I'm proud to have been there at beginning, helping people to understand this, see the potential, and adopt it as a technology.
I'd try to impress upon them how incredible such a system is, compared to every other record-keeping method humanity had developed to date. It will be difficult for them to comprehend this, since in their lives, authoritative decentralized records of all sort will just be a longstanding fact of life that they've always lived with. No need for notaries, stock exchange management entities like NYSE, clearing houses, etc. They'll just be used to publishing transactions/contracts/whatever to some crypto-system and that's that; no middlemen, no delays. It'll be obvious in hindsight that these things reduce friction and expense, and is the rational way to go about all manner of financial transactions and contracts.
Along these lines, I'll end with Naval Ravikant's excellent post outlining the beginnings of all of this; hopefully all of which will be reality long before for my grandkids are even born:
The Internet of Money:
http://startupboy.com/2013/11/07/bitcoin-the-internet-of-money/