When my kids were young, I insisted they read one classic (what we call a "core collection" book in the library biz) for every two or three series paperbacks they read. As tyrannical as this may sound, my kids actually did not require much governance on this one - any child who is worth his or her salt as a reader comes to recognize rather quickly what constitutes an absorbing story. They so often chose truly interesting and well written books that I ceased to have to monitor the situation. We saw the series paperback as a sort of "rest period" between more challenging books. The idea is not to eliminate series books which kids obviously enjoy. The object is to challenge your children's reading ability by carefully evaluating the three Ss: the serialized, the standardized and the superficial.
Series paperbacks are books built on a basic formula: a character or group of characters performing a type of adventure or action, such as solving mysteries, playing sports, etc. The stories can be predictable and somewhat repetitive and demand little of the reader. They're not meant to be great works of fiction; they can be entertaining but not particularly thought provoking: a light read. Stories are often pleasant, and can teach positive values, or they may be silliness for the sake of it. If your children are unwilling readers, these may ease them into reading. Thinking back, I wonder if my enjoyment of comic books was inversely proportional to the disapproval of them by grownups.
Beware of the occasional series, adult and juvenile, where the big mission seems to be to crank out little money-makers as quickly as possible. I suspect that certain authors have a database set up which works something like the old Mad Lib books - the author fills in new character names, place names, verb here, adjective there, etc. and the database produces a "new" story. Admittedly, reading them is better than not reading at all. But sometimes not by much. Children's reading skills need a workout just as their bodies do.
When a book series becomes too standarized, it fails to activate a child's imagination. Have you ever listened to a band and thought that their songs all sounded kind of similar? Or watched a sitcom and felt like you had seen it before (or something very like it)? As the old joke goes: "I listened to a country song backwards, and my wife returned, I got the house back and my dog came back to life." Sometimes genres fall into a malaise, writing the same plot over and over with minor variations. It's as if they all attended a writing school in which they were given a set of plot parameters and told never to vary. If your child can guess the ending by the end of chapter one, they may not be sufficiently challenged.
There is apparently a fine line between elegantly simple and superficial, and some publishers are missing it. The library has a superabundance of books that propose to teach: ABC books, Count-to-Ten books, Name-the-Color books, etc. Some are sweet or clever or ingenious while some are lame and spiritless. Little does the average person suspect that they, too, could be a published author even if they have never picked up a pen. A (insert picture of Aardvark), B (insert picture of Bee), and so forth. As long as you can spell Umbrella, how hard can it be? Even if you are reading to a toddler, you don't have to bore them witless: after the first reading the child pretty much gets that C stands for Cat. There are too many authors who lean on a filler formula established in the misty past: teach children shapes, colors, ABCs and how to count to ten. Who cares if they're having fun? Almost as bad are the books which replace plot with repeating major characters who have lately arrived from certain daytime cartoon shows. Let's just say that brand recognition does not a true literary experience make.
Whether you are reading to your children or they are reading to themselves, a book should be well constructed enough that some element of it sparks the imagination, initiates a thought process, provides a surprise or an insight, or gets a laugh. Books are a window into all the great ideas that have been passed down through time - noble thoughts, beautiful places, character building experiences. Encourage your kids to choose the window with the view.