I've done sand tray work myself and I have done it with children and I think Ellie and Skysblue have described it wonderfully. For me the thing is not to over think it. Just go with it and it may be that what you make means nothing to you but later you may notice something about it or themes that emerge in several trays. Children really make the most of it and although some take a while to have the confidence to use it once they do they don't tend to worry about reading into things, they just play. It is an amazing healing tool for the mind. The subconscious tends to do its stuff whether you want it to or not.