One of the most helpful resources in my library is a short book called A Simple Way to Pray by Marin Luther. It was written in 1535 for his barber, who was struggling with how to cultivate a consistent and meaningful prayer life.
Here is a link to the pdf: A Simple Way To Pray .pdf
R.C. Sproul and the team at Ligonier took this book and produced a wonderful new story aimed at younger readers. The audio book was recorded with Dr. Sproul as the narrator. I recently played it for my kids and they thought it was great.
You can download a free copy of the audiobook here.