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.