There is no shortcut to improving children's English! Work with a clear objective in mind, understand the children, their strengths, their interests and their moods.
Don't base your selection solely on cost and children's English level. Think about their age and interests.
Check the substantial range available for any English Activities (Eigo katsudou) program!
Sometimes a good melody and rhythm can help children to input new language in a fun way.
Chidren need to feel the moment of "Wow! I can read!" using text outside of the textbook.