Little Bird Tales is an easy to use digital storytelling tool that can be used online (they do have an app also that is $2.99). Students can upload their own artwork, record their voice, add text, and can be shared by purchasing an mp4, emailing, embedding in your classroom website, or creating a PDF.
Here's how it works:
You could set up accounts for individual students and have them write stories...
or create class books, like this example:
They also have Common Core templates build in for grades K-5 when you set up a teacher account!