D.H.
I'm very sorry about your cat. We went through the same thing with our cat last summer. Our situation was slightly different though, because we didn't know for certain whether we were going to put him to sleep that day. Our kids (who were 2 and 5 at the time) knew before we took him in that our cat was old and very sick and that he could die soon. When the x-ray showed that there was nothing we could do, we decided to put him to sleep then. So when my husband came home without the cat, we just told the kids that he was too sick for the vet to make him better and he died. We didn't discuss how he died, although if my son had asked more about it, I was prepared to be honest (but I would not use the term "putting to sleep"). My husband and I were much more upset about it than my kids. So I would recommend keeping explanations vague, and just answering questions as they come. We didn't do any kind of formal memorial, but we do have some pictures of him around. I think a photo book is a nice idea.