Amazing, amazing, amazing. I couldn't put it down. For fourth grade and up. Did I say it was amazing?
This was not my most favorite book. (The Lemonade War by Jacqueline Davies), but Lucy loved it. Mary really liked it too. It's about a war between siblings. Interesting math and economics in it as well.
Give this a shot (Gooney Bird Greene by Lois Lowry) for your younger girls and boys. Read aloud or by themselves. Pretty funny. Engaging. Sweet.
And while you are at it, try this for some great short stories. Funny. For kids first grade through fourth, boys and girls. Really cute
(The Magical Mrs. Plum by Bonny Becker.)