A specific Main Teaching is needed not only in your lesson, but throughout your entire children's church hour in order to drive home a basic point or Bible doctrine.
It is commendable if you desire to cram as much Bible into your kids as possible, but... quite frankly, if you have too many points or ideas, they often don't "get it." However, if one particular statement is repeated throughout your song service, puppet dialogue, lesson, skits, and other activities... chances are they will remember!
So how do you decide what Main Teaching to use? The best results will come from leaving that decision up to the Holy Spirit. Pray about the needs of your kids, and pray for all of the children by name on a weekly basis. The Lord may point out a need that you didn't even realize was there. You will find that from prayer and seeking the Lord's direction, your entire ministry will turn upside down -- your kids will behave better; you will begin truly teaching them something and seeing them grow spiritually; and you will develop a true heart of love, ministry, and service toward them!
When you feel God's leading toward a particular topic, begin to develop your lesson and other children's church components around it. Search for a Bible story or doctrine that will allow you to drive this thought into their young hearts a number of times. Try to plan everything around this theme: the songs, games, skits, memory verse, stories, and so on.
This Main Teaching needs to be wrapped up in one simple statement that can be repeated throughout the children's church hour. You need to try to state it with the same wording each time so that it really rings out in their little brains: