Pause from time to time. It'll keep kids "on the edge of their seats!"
Think of ways to build tension for the kids.
Have as much talking between the characters of your story as possible. Instead of telling about something happening, have your characters talk it out between them! Take a lesson from the soap operas, which are day in and day out, the most popular shows on TV. They tell about the happenings, by the characters in their story telling each other in dialogue!
Have a character sound like what they're saying.
Unlike TV and Movies, which use music to set the mood, you usually don't have that at your disposal! But don't despair! That doesn't mean that you don't have things at your disposal. All that means is that you have to be more creative that just relying on music!

Use voice inflections whenever possible!
Use sounds effects. They don't have to be involved. Simple sound effects can be made with your mouth. Like doors, (Which almostalways creak in stories) sirens, wind, etc. More complex ones if you're talented that way. (I"M NOT!)
Practice them at home. Don't think that they'll somehow come out when the time's right. If your wife or yours husband thinks you're loosing it, then go for a walk in the woods. Although, they might as well get used to the fact that if you're a successful children's minister/teacher, then you're goofy!
In order to continue on with storytelling, click on 3.