1- The magician hat design is difficult to use on small phone and there is plenty of empty space left on the screen.
Main problem is the size of the stars to press on. Good news is that there is enough place to increase the size of the stars.
2- Also to allow to use it for a pupil who does not know how to read, it would be nice to replace the initial text "click on the hatto begin the game" by an icon telling to first click on the hat.
3- It would be nice to take the stars under the hat and allow the user to compare the expected number of stars with the result he gave before to say if it is right or wrong. This start the correction process in the user mind and he can build his knowledge on the error understanding.