I can say without a doubt movies made from popular novels can increase desire for reading as a child. It is an incredible way for one generation to share a beloved story with a younger generation in a way the younger generation will be able to comprehend. There is also the ability to share a movie with the child and explain how it might be different, and inspiring curiosity in the child to read and find the differences.
With so many popular stories becoming movies, there is no reason for children to pick up a book and read it. We live in a fast-paced world where instant gratification is wanted in most situations. Movies that only last one to two hours allow children to see, hear, and understand a story in a much shorter time frame than reading a book. It also does not force them to use their imagination and therefore is less work for those of more lazy society.
I think that making popular children's novels into movies actually decrease a child's motivation to read because they will think that they can simply skip reading the book and watch the movie. I think that adults cherish the opportunity to compare the novel to the movie but I think that kids today want to take the easy way out and simply want to watch the movie if they have the opportunity.
Turning novels into movies just fosters the idea of valuing movies over reading in children. Why would a child decide to read if they could just download the movie version? This happens with adults and children already. It is so common to forgo reading for the movie or TV show today, and the screen adaptations are usually horribly wrong, anyways.