Shinies are the spirits of children that have already crossed over into the light and they are powerful and strong. Aiden, Melinda's son, is the only one who can see them. There are many more Shinies than Shadows. According to Aiden, "If there was a war between them, the Shinies would win."
Eventually, that war came. Carl, The Watcher, was the one who told Aiden that he was meant to bring the Shinies together to rescue Melinda from the Shadows.