The Clue at the Zoo — Patricia Reilly Giff

Oh, no! Arno the killer kindergarten kid is back. And Dawn Bosco's grandmother is his baby-sitter. How will Dawn and Jill ever have fun at the zoo with him along?

Luckily, Dawn. the Polka Dot Private Eye, finds a mystery to solve. Someone has lost a notebook with gritty white poison all over it. But Arno has disappeared. If Dawn and Jill spend all their time looking for Arno, will they ever find out who owns the book?