Shopping for children's books can actually be fun, and book ownership can be immensely rewarding for your children. Don't insist that every book be of great literary merit. Many children go through a phase in which they read one type of book to the exclusion of all others. These are most often series books of some sort, but there are children who fixate on fantasy books, or animal stories, or Tintin titles. These phases cause great parental concern, but they generally pass.
Most crazes have more to do with peer pressure than anything else.
I asked a girl who religiously buys each Goosebumps title as it comes out whether she thought the latest titles were as good as the early ones. I just get them for my shelf. While books might deal with difficult situations, they can help children to make sense of the world around them. To reject a book because a character's parents are divorced or a beloved grandfather dies robs a child of the opportunity to imagine what she would do in that situation, and to see how someone else handles it. Don't stop reading aloud together when your children have learned to read for themselves.