The story of 'Bridge to Terabithia' is quite suitable for kids. It shows the power of friendship between the two main characters, Jess and Leslie. The fantasy world of Terabithia they create together is full of wonder and adventure that can engage young readers. However, it does touch on some sadder themes like death, but it handles them in a way that can help children understand the concept at an appropriate level.
You can try websites like Project Gutenberg. It offers a wide range of free e - books, and there's a good chance 'Bridge to Terabithia' is available there.