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 love this answer, this also applies to your bank accounts and other things you only have the password for, make arrangements for your heirs to have access to your stuffs after you are gone,...
As much as I don't want to talk about this matter but this is reality. Yeah I like that idea about putting it to your last will that if you have lots of investments and accounts. But to those who only have enough, it would be best to share your passwords and other important and private matters to your better half or to someone whom you trust the most.