Children's Sermons Should Be:
It should be a Biblical children's story: The theology for the Sunday Morning Kids Story doesn't need to be sophisticated but it does need to be Biblical. Keep it simple but keep it scriptural.
It should be an Interactive children's message. Don't preach to the children. Involve them in the learning experience.
It should be a short children's message. Most kids have a very short attention span. Don't let them get bored. Otherwise, the main thing they will learn is that church is boring. I would keep it under 5 minutes.
It should clearly present some aspect of the Gospel. If you get too abstract most kids will miss the point. Keep the thought progression simple.
It should be a FUN children's message. Ideally the children should look forward to this time in the service.
It should be a "risk free" children's message: Protect the kids from being embarrassed in front of the congregation.