Lee Strobel has helped many an adult grapple with some of the toughest questions ever in apologetics (why believe). Now his thoughts have been repostured for kids in a series of three books: “The Case for a Creator for Kids,” “The Case for Faith for Kids,” and “The Case for Christ for Kids.”
You could do much worse. With just about any other book on Amazon (except the Bible). These books will help your inquisitive youth find answers. Highly recommended.