Need some help using AppInventor by MIT?
Use this great set of tutorials, featuring interactive quiz questions for every tutorial.
Featured applications to build:
Text to speech
Painter Pro
Camel Mash
Times Tables
Bounding Ball
All feature a fully working application that can be expanded to make an App that would be Google App store ready.
Completely free, and supported by a set of teaching resources at: https://www.tes.com/teaching-resource/appinventor-computing-programming-scheme-of-work-for-ks3-11330823