From an English teacher's point of view, I immediately think of one good reason for and against adapting children's literature into other mediums such as plays and movies. First, let me say that anything that gets kids to read, I am supportive of it...including the Captain Underpants series. Having said that, it brings me to the biggest disadvantage of producing alternates to the actual books: If the child can watch it on TV, on stage, or in the movie theatre instead of reading it, most kids today will opt for that. It is instant gratification and doesn't take as much time as the actual reading. So, they get the story, but they don't get the discipline, the language, the sentence structure, and the overall benefits that reading brings. Research proves that the more you read, the better you write (hence the exposure to more words and the correct sentence structure on the page), and the smarter you are in general.
On the other hand, seeing the story on film might inspire kids to actually read the book. Or, the movie/play could be used as an incentive to get kids to read first then go see the film, etc. as a reward. It is always good to compare the two afterward, since kids will almost always see that the film can never be as good as the book. Take, for example, the Harry Potter stories. The films are excellent in terms of special effects, etc., however, you can not effectively mash 300-500 pages of material into a 2-hour film. Something pivotal will be left out, rearranged, or otherwise destroyed or altered. The book allows time for readers to consider what they would do in the character's shoes, and it allows for the character's thoughts and motives to be played out in a way the stage and screen are lacking. In addition, seeing the film/play through a director's eyes does take away from the imagination side of it. I can't tell you how many times I have been disappointed that the main character on film looks completely different than how I pictured him or her.