As Castle episodes go, “Child’s Play” may not move the key story lines forward much, but it sure is a fun one to watch! Castle (Nathan Fillion) going undercover in an elementary school? It’s like putting a duck in water. In “Child’s Play,” an ice cream vendor is murdered and during the preliminary investigation, Ryan (Seamus Dever) discovers a field trip permission slip belonging to a second grader at Kennedy Elementary who may have witnessed the crime.