Some usefull information can be found here https://news.bitcoin.com/how-to-bequeath-your-digital-assets-to-your-descendants/

Its vital to keep your private keys in a safe place. For example you had 100k worth of bitcoin on site A and this happened to go down or vanish over time..With private keys you can retrieve your coins from the bitcoin network into another wallet or site B. Bitcoins are always safe if you keep your private keys safe from hackers or greedy family members.

My advise is buy a mini saftey deposit and place you private keys inside. Instruction with the deed lawyer. Lawyer dosent need to know what asset are being written down. if tax has been already paid no need to worry!. some countries do require inheritance tax to be paid. In the bitcoin law im not to certain.

Trezor is another safe device..the issue is firmware updates are required on regular basis.

The other method is time lock bitcoins. :-)

Regards