Death portraits used to be perfectly normal, on film and in paint and in the form of death masks. I've been reading _Kodak And The Lens Of Nostalgia_, which is basically a history of early Kodak advertising, and there's an interesting sideline on the clash between the tradition of the death portrait and the idealized photographic view of life that Kodak's ads promulgated. If not for the "Kodak Moment" concept, we might still be taking them routinely.
I think children's deaths are a special case, since the nature of the parents' loss is *so* different from other circumstances. It seems like a lot of people find the portraits help them to cope with their grief constructively, and it would be uniquely insensitive for people to tell them not to do that because it's too morbid.