This also applies to children with any sort of problem, from nearsightedness to learning disabilities, being forced to be a party to a discussion about what books might help with that problem. One of my worst moments as a bookseller came when I was approached by a mother accompanied by a very large, very unhappy-looking teenager. The fate of the civilized world doesn't depend on whether or not you always make the right book-buying decisions. 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.