Ask yourself this question : How are your other assets and wealth being handled at the moment? You use a Will, right? I would suggest that you make it a riddle that only your family would be able to understand and put it in your Will.

The Passphrase could be the answer to a question that only your family would be able to answer. Example :

~ Where was our first family holiday.
~ What was the name of our first dog.

You also hide the paper wallet < Metalstamped seeds/keys > in a location only they will know about.

~ Lemon tree at the orchard.

Do not use safe deposit boxes, because they are raided regularly by banks and thieves.