As part of our religion experience this year (I hate to call it a class - after all our faith should permeate all that we do), the boys and I are using Following Christ, the 6th book in the Faith and Life Series. Chapter 3 was on prayer. Now, whole treatises have been written on prayer. I've read quite a few of them. This was just a chapter, but it was a good one and very age-appropriate for my pre-teen children.
I especially liked this quote by St. Alphonsus Liguori which is a good reminder for all of us, regardless of age:
Form the habit of speaking to God as if you were alone with him, familiarly and with confidence and love, as to the dearest and most loving of friends. Speak to him often of your business, your plans, your troubles, your fears - of everything that concerns you. Talk with him confidently and frankly; for God does not often speak to a soul that does not speak to him.