I was actually thinking recently - this would be an interesting place for John K to branch out, and maybe finally form a proper company, since he already holds so much in escrow and has trust established unlike any other member of the community. Since he's quite knowledgeable in Bitcoin, he can offer a Q&A session with family, whereas a letter can't do that and you need to have everything either fully explained to them while you and they are both alive, or a pretty darn good (long) explanation in text.
You would pay someone like John to both hold a key fragment and explain to the beneficiaries what Bitcoin is and what their options are. Of course, if you don't go through someone like John, you might not expect them to realize the massive death taxes they'd be liable for in most countries.
From my private discussions with him, John K actually has a lawyer, for in case he goes, the lawyer does the escrow.
For this particular purpose, as soon as you can, educate your children/beneficiaries about bitcoin. Get them to use it. Go buy a ticket from my lotto. Go to bitcoinstore.com and buy a computer. Go to bitstamp to exchange.
Then, once you know they understand about bitcoin, tell them you have a dead man's switch for their current email address with their current GPG key, because you will surely send or compose it encrypted, and/or you have a sealed envelope at your vault just for them containing a cold wallet private key.
They know what to do with it.