When faced with a possibly life-ending diagnosis, I wrote all my children letters. I included things like their first words, favorite memories, what made me the most proud of them and added a few things about the cruelty of the world and how to get through those times.
I also wrote my favorite foods, books, places and things to do as well as past illnesses in case they ever needed to know.
I guess it has a lot to do with how long someone intends to be gone as well as their ability to write or talk to their child during that time. In my case I thought it would be permanent so I wrote everything I could think they might want to know someday and included a few of my favorite photos of me and of them. I sealed them in an envelope and told others to give it to them when they thought they could handle it.
Thankfully, I lived and I now own those letters. Someday when they are older I still plan to hand them out.