For parents, the scariest thing about Halloween is the lurking question: When kids choose their Halloween costumes, are they choosing their alter egos — or revealing their true selves? In two new picture books, “Crankenstein,” written by Samantha Berger and illustrated by Dan Santat, and “Zombelina,” by Kristyn Crow with pictures by Molly Idle, kids really are monsters and zombies — and not just on Oct. 31.