Granny by Anthony Horowitz

Twelve-year-old Joe Warden isn't happy.
Sure, he's rich, but his parents don't care about him. His grandmother should make everything better, except that Joe's granny is a nightmare. She's not just physically repulsive, she's horribly mean.
Everyone thinks she's just a dotty old woman, but Joe knows the truth. He's seen behind her mask and glimpsed the wicked glimmer in her eyes- she is pure evil! And now she's out to get Joe, unless he can stop her and her band of nasty grannies first.

Sixth Grade Secrets by Louis Sachar

When Laura starts a secret club at school, she makes the other members give her something totally embarrassing as "insurance," to make sure they won't tell anyone else about the club. She promises to keep the insurance secret, unless someone blabs. Gabriel, who never tells on anybody, would be a perfect member of Pig City. He wants to join, too. But when Laura asks him, something goes terribly wrong. Now there are two secret clubs in Mr. Doyle's class, and guess who's the head of the new one--- Gabriel! Each club is determined to topple the other one, and nobody's secrets are safe!