The challenge is on for young amateur sleuths: how do their skills stack up against a brainy pre-teen who helps his detective mom figure out “whodunit?” Ever since Jonah Bixby’s father—also a detective—died in the line of duty, Jonah would come down to the station house after school to wait for his mother to get off work. All the cops took him under their wing, tutoring Jonah and turning him into their unofficial mascot.  In the process, Jonah became a crack crime solver, and these 20 cases, from “After School Homicide” to “Vampire on the Balcony,” are among his most fun and intriguing. They include missing objects, jewel thefts and art heists, murders, accidents, and even some pranks.