Now that you've got some actors, it's time to come up with a plot. Don't go crazy; choose something short and simple. If you're working with kids, help them brainstorm ideas. For young kids, it may be easiest to have them choose their favorite (short!) book to act out.
Write out a "script" that includes details about what the characters should be doing at any given time. If necessary, break the story into scenes, where each scene has no more than one background/set.
My favorite first film for kids is to have an object move into view in the scene, travel across the set, and exit from view again. This does well with a toy boat or animal as an actor. It's simple, quick, and is enough to get kids engaged. It's also a good way to get a sense of how much you have to change an object in each shot to get realistic movement in the final film.
