Well, if you have a substantial amounts of bitcoins which can effect your heirs life in a positive way, I will recommend you to add it to your will. Don't mention the private key to your will, instead store the details of your bitcoin wallet in a secure location in your laptop or PC or in a piece of paper and mention that location details to your will, so that your heirs can find it after your death.
Else, have some faith on your wife and tell her everything about your bitcoins. Let her know how to access it and ask her to distribute it among your children after your death. But if you are holding a very small amount, then it's not worth having the bitcoin details included in your will.
I think this is applicable if the person who died would actually leave his holdings to a person or would form part of his will. However, in the event that a person dies and none of his family or heirs for that matter would not know of his holdings, then it would just remain in his wallet until the time comes that it would be irrelevant to the circulating amount of bitcoin in the market. At first, it would play a part since bitcoin has a market cap, and that bitcoin in the person's wallet has surely been counted as part of the cap, which means, if he holds a big amount it would affecting the circulating amount that might play a part in making bitcoin's value higher since the supply would be lower. However, after some time, when the circulating amount diversifies, then the amount that is left in the wallet would be rendered insignificant due to being idle.