Talk to your local librarians and tell them about your idea. Many public libraries are open to having volunteers lead their story times, and adding an unusual twist, like presenting a storyboard, will make the experience all the more fun for the children.
Find out how many children usually come to your library's story hour. Because storyboards are generally made up of a series of very small pictures, presenting it could be difficult if you have a large audience. You might want to bring along a projector to increase the size of your display, or digitally scale the images and print them with one frame per page. If you take this latter route, you could still paste several pictures together onto poster-board to maintain the storyboard feel.
Present your boards to the children in a clear and entertaining way. Explain what is happening in each frame, but leave out technical things like camera movement or transitions: these will only confuse your young audience. Talk in different voices for the different characters' lines, and narrate enthusiastically.
Pay attention to your audience's response. Especially if this presentation is meant to gauge children's reaction to the story of your eventual film, noticing when parts of the story seem to confuse or bore the audience is essential. Observe which parts they find funny, as well: children are often amused by things in which adults find no humor, and vice versa. Playing up these moments in your final project will increase its chances of success.