OK let me start with the last point first, there has been a whole argument on this subject four years ago, which I am not going to retell in detail. Basically it was highlighted by many authors how inappropriate age certification on books could be. If you are a struggling reader aged 14 do you want to be seen reading a book which has 7+ plastered across the back. In the same way should you be prevented from reading above your age if you are a confident reader. This is the time for booksellers and librarians to step in and use their expertise to guide potential readers. Luckily, Patrick Ness was there to rebut this idea and point out that it would be irresponsible 'for young adult novels to ignore the darker side of life.' As a writer I was slightly concerned when GP Taylor stated that he hadn't really read Vampire Labyrinth when he wrote it. I know when I edit and rewrite I am constantly reading my own book. I need to know it is working and to take a step back from it, reading it as a reader and not just as a writer. But maybe that is a personal thing.
Going back to Kathy Short's points about abandonment, alienation and homelessness being themes in modern children's literature. Firstly, I would suggest they are there because they reflect the world we live in. We live in a world, rightly or wrongly, where our children have access to a lot of images and ideas via the media including the internet that we probably wouldn't have seen as children. But I would also contend that they are not new. What about Mary Lennox in Frances Hodgson Burnett's The Secret Garden (1910). You could argue to some extent she suffers from all three of these. Then there is Roll of Thunder, Hear My Cry (1976) by Mildred D Taylor, now there is an amazing but truly dark book at times. Or even William Golding's Lord of the Flies and numerous Roald Dahl books. And these are just the few that came immediately to mind. Children's literature is often dark but invariably offers hope by the end.
As a writer these ideas can be plot devices allowing your characters the freedom to make their own decisions and solve the problems they are in without relying on an adult. The 'absent parent' can take many forms. For example, it can be the alcoholic parent who is there yet absent. It is what helps to make a good story and that's what children want. A good story that they can get lost in. They will be looking for characters to identify with; characters like them but also, and equally important, characters that are NOT like them. It is all part of the search for an identity.
Yes, there are lots of dark stories out there but there are also a lot of other 'lighter' stories which are not being mentioned (this is not meant as a derogatory term but just to infer the opposite to dark stories). Children and young adults will read all sorts of stories. Some dark, some not. They will read what they need to read at any particular moment. And I would suggest that it is up to us as writers to continue to write good stories, well told whether they are dark or not.
Going back to my childhood, a bit of Paul McCartney