How should I write a bed time speech for children?

I think that bedtime speech means the same thing as bedtime story.  You can prepare your speech in the same way that you would prepare any other speech.  Before beginning, you must consider two main things -- your message and your audience.  As the question mentions, your audience is children.  Next is your message.  It's a story, but what is the story about?  If the child is a girl, perhaps something about a princess.  It doesn't have to be a princess, but I would make the protagonist a female.  If you are speaking to a boy, then make the main character a male.  If the child is young, then the story line is going to have to be short and simple.  They simply do not have the attention span to last for much beyond ten minutes.  Based on my own three children, the story should be exiting, but not scary.  Remember, the child is hearing this before bed.  You don't want your story to cause the child to lay awake in fear for the next hour.  Keep your language simple, and use a lot of color in your narration because it will help with visualization.  Some people feel differently about this last part, but I like to end bed time stories with an actual ending.  Bring the story to a close rather than leaving it with a cliffhanger.  Again, you want the child relaxed and ready for sleep.  You don't want them continuing to think about the story until very late at night.