Read the article at www.livewritethrive.com/2012/01/18/yes-you-do-need-a-theme.
Now think of some classic children's books and their opening lines, then make some educated guesses about these stories' themes. Some classic children's books you could start with could be: "A Wrinkle in Time," "The Secret Garden," Little Women," "The Adventures of Alice in Wonderland, "Tom's Midnight Garden," "Treasure Island," "The Adventures of Tom Sawyer and Huckleberry Finn," the Harry Potter books, and so many more. Now go and find some others, and discover their themes in their beginnings. Have fun.