Frindle by Andrew Clements. Pub. Simon and Schuster, 1996.
Aimed at ages 9 through 12 your kids will love it and you will too.
Nick is in fifth grade and he is the witty boy in the class, always with a unique angle and with something to say. When his teacher, Mrs Granger, is discussing how words enter the language and the dictionary, Nick decides to call a pen, a frindle.
After using the word several times Mrs Granger gets fed up and bans the word.
What a mistake. The next day the whole class is using it, then the whole school and then it is banned at school. Newspapers and TV find out about it and the whole scenario explodes from there. Soon there is the T-shirt and an industry built around the frindle.
Easy to read your kids will love you for it. Check out his other books too they are all good.