You can just write the private key in a letter, and seal the letter and gave it to a lawyer who will give it to your children when you die.
there is no way I would trust a lawyer with this, he opens letter and burns it after. No one would ever know!
Split the private key using Samir's Secret Sharing algorithm and give several trusted parties a fragment. For example, a 3-of-5 scheme where you give a fragment to a spouse, friend, family member, lawyer and bank deposit box. Any 3 fragments can be used to recover the private key. A short instruction on what Bitcoin is and how to use the private key you can spread more liberally to your beneficiaries.