I make a point to talk about John a lot with the kids. I'm always saying things like, "Daddy loved to go to the park!" or "This was your dad's favorite shirt," or "Look, that's a picture of you and Daddy when you were very little." I am especially aware of showing the kids pictures of John. I don't know how their memories work, but I suspect that they don't actually remember John. They do seem to know who he is because of hearing me talk about him ad nauseum.
None of that takes away from the impact of hearing Riley gleefully say, "Daddy!" when he sees a picture of John. We have a little "Who Loves Baby" book into which you insert your own pictures; the first spread in ours is a picture of John holding hours-old Maddie facing a pic of Dinner a few weeks before I had her put to sleep. Riley loves to look at those pages and point and say, "Daddy! Gato! Daddy! Gato!" Then he closes the book and says, "Bye-bye. Bye-bye."
It gets me every time. Meaning multiple times a day.