While searching for apps to use in my classes I found tons of free ones. There is definitively a wide variety of apps for different levels. Some of the free apps might be a sample of a complete app, but it's worth the try. I do recommend that you download the app and play with it before using it in class.
Here are some of the free apps I found:
Disclaimer: Make sure to download the app and explore it before using it in your classes to make sure the content is child-friendly.