Many of the popular kids films are based on story books and novels. Their curiosity and interest should be sparked after viewing the film. Many kids prefer to read something which, in their opinion, is safe. In other words, something they are guarenteed to like. One benefit of reading a book they've previously enjoyed as a film is the familiarity of the characters.
Kids like to know what they're going to get.
Characters they already feel sympathetic towards are a necessity in reading enjoyment. It's like taking a short cut. They'll want to read about characters who stir their emotions and face seemingly impossible odds. They can also easily imagine the characters in their minds. Having seen them already on film, the characters will become alive much faster.