I have two great finds to share, written specifically for kids to help teach basic money principles.
Rock, Brock, and the Savings Shock (Money Tales) by Sheila Bair is a picture book aimed at 4-9 year olds. Rock is a spender, Brock is a saver, and Rock looks on over his pile of stuff while Brock ends up with over $500. It's great for teaching kids the value of saving for big goals instead of squandering on little things.
Make Your Own Money: How Kids Can Earn It, Save It, Spend It, and Dream Big, with Danny Dollar, the King of Cha-Ching by Ty Allan Jackson is an easy chapter book aimed at 8-12 year olds. It goes into how money can be earned and saved, and it does a bit of investments, too. It's extremely approachable for youngsters just learning about money.