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.
Yeah, anything can be WILL to your offspring, but adding bitcoin demands that you have a certain degree of trust and understanding with the lawyer managing your will.
Your bitcoin assets are under the jurisdiction of anyone who knows your bitcoin wallet's seed phrase. Anytime he wants, he may exchange it.
What I would do if I were to include bitcoin to my offspring, what I will do is purchase a hardware wallet, give it to the lawyer, and give my children access to the hardware wallet's password. My children should meet the lawyer whenever I am no longer alive because I will specify in the WILL how the bitcoin in the hardware wallet is to be shared