Our 6 year old, Bethany is sitting with us in the Sunday Morning Worship Service. Last fall, my hunny and I attended a seminar about this very topic. The speaker was informative and had some good ideas for helping children learn to worship God and give back to Him during the service. She had an article published called Worship Wiggle Busters (Christian Parenting Today, Nov/Dec 2000). You can read the article online here.
I'm in the midst of making Bethany a notebook--part of it is her prayer notebook and the other part will be her church/sermon notebook. Her prayer notebook has the week divided up and she prays for different things each day. I've included pictures of family, friends, pastors, US leaders, etc. as well as pictures for things we are thankful for so that as she prays she can direct her focus. I'm also trying to figure out a way to put pictures of different attributes of God, so that she can express her praise and worship to God for who He is.
The sermon/church part of the notebook will have a place for her to draw words in the sermon that she understands and as she learns to read and write, she will also begin to write words and sentences.
Some other books that you may find helpful on this topic:
Parenting in the Pew by Robbie Castleman
The Hidden Art of Homemaking by Edith Schaeffer
I also remember hearing Elisabeth Elliot recommend practicing at home by listening to a sermon CD or on the radio. Have your kids sit with you on the couch, listening as they would during a sermon.
I'm going to get busy on Bethany's notebook, so I can show it to her before Sunday. What a fun way to prepare to worship the Lord together with our church family.