1) It's relatively easy to come up with a plausible scenario for a movie. We're great at complicating things. It's much much harder to create an ending to match. The list of movies that concoct a fascinating scenario in the first hour but flame out in the last forty-five minutes is very very long.
2) Child actors are like poetry. Some are just watchable, most are boring, but when they're good, they're often incredible. 
3) Justine Triet's French legal drama Anatomy of a Fall does two things many movies struggle to do. One, it builds a complex, riveting plot and follows through with an ending to match, and two, it delivers that ending via a superb performance from a child actor, Milo Machado-Graner. The movie does, of course, also lean on excellent lead actors, notably Sandra Hüller, as well as crisp directing from Triet, and even an extended instrumental version of 50 Cent's P.I.M.P., but props to both the scriptwriters and to the exceptionalism of Machado-Graner, without which the movie, however lofty and tense the setup, doesn't land.