You just said a will can be written, WILL contains everything you have, properties, money and so on, if money can be included why not Bitcoin? Anything can be WILL to your offspring.
You don't have to hide your involvement with Bitcoin from your children and wait till you are no more around before they know, teach them about Bitcoin while you are still here and pass them the Bitcoins you want them to have, there are ways you can get this done, I believe we have discussed something like this on this forum.
If your kinds are still very young, wait for them to get to a certain age, you have a wife? Teach her if you trust her, you have to do so because you don't know when your time will be up, if you are not around anymore she will get things done and fulfill your desire.
Get your kids hardware wallets each and send any amount, for example 1 BTC each or lower.
Lawyers can get this done, so is your spouse if you are scared of Death 😁😁
Well, you have spoken well but from what I could derived from op statement writing a will and inform of bitcoin phrase and hand it over the your lawyer or his lawyer is something unimaginable, why because those asset being bitcoin can be stolen by even your lawyer or your wife without knowing since bitcoin address doesn't show or displays recipient name. To say it's your lawyer or your wife, unlike other WILL's that is being controlled by Third party (Money is easily to managed and WILL to their kinds, which involved the bank and your Lawyer and your wife) So, there is no way bank could initiate any transaction without your wife, your lawyer to take records of all every that is happening. Bitcoin can be easily stolen, lemme just break it down without any hidden truth.
Do you know if op is already 70 years and his kids are still tender in a way they can't handle bitcoin transaction or they don't have the knowledge of bitcoin. Naturally the best is to teach just as you already said but what in a way those kids can't manage funds properly and this keys (Phrase) are being handed to them how do you think those sets of kid can be able to manage those funds and can they secure their phrase properly because including
Third Party is putting funds at a very high risk to say.
What about spouse?
Yes, there are some wife who doesn't care and they always wants their share and sometimes they alway create issues haven known that the man is super wealthy, they could file for a deviors in order for them to have their wealth. My question is, do you still care about such wife giving her your phrase to store for your kids?
The answer is simply know because such person is only wanting to have all control of your assets to her way, what about including a family relatives, I think is another huge problems because the death doesn't speak after laying like a log of wood, and you wouldn't know how those funds are being managed at your absence.
So the best is just to keep eyes on your children for them have the basic knowledge and then come up age that is the only way possible.