I sit down with a couple of the pastors in our church and look at what are some of the important stories and subjects that are most appropriate for a Children's Sermon.

Not every story or topic is best in Sunday School, and not every story or topic is great for a children's sermon. 

For example:  teaching about worship and prayer are great to do in worship. But teaching about involved stories, like Exodus, are best left to teachers in rotation.

Hope this is of some help.
Best wishes to you in your ministry,
Eliana