Watch Mystery Kids for Free: How to Stream the Movie Online at 123Movies. . The surprisingly intricate, nimble plot is about a couple of kids who play amateur detectives one summer in the hopes of collecting a $5,000 reward offered to anyone able to find a missing 17 year old girl. The girl and boy are best friends who have a great, unforced camaraderie with each other. None of the tiresome sniping and bickering that goes on in so many other films of this ilk. And they are given some wonderfully smart, observant dialogue, tons of it. In fact, the film is overflowing with it, much of it tricky and the two actors handle it all like a couple of seasoned pros.