The Central Coast Children's Foundation (CCCF) in Monterey California has put together a very useful list of Literacy Apps. While aimed at Adult Literacy the list also includes a category of apps that can be used with young children who are beginning to develop their literacy skills. The list provides details of the App and what it does as well as the costs, platforms it can be used on, and any reviews available. The following link takes you to the downloadable pdf on their website.
Adult literacy Apps list.