Having never read the novel by Tom Rob Smith, I can't really comment on "Child 44" as an adaptation. Often, even I feel like a movie doesn't work, as long as it resembles the book, fans seem placated, and that might be the case here. Coming to it cold, though, my reaction is one of bewilderment. I have no idea what movie they thought they were making or why people flipped out for the book, because there's nothing in this film that would suggest a compelling story compellingly told.
Tom Hardy stars as Leo Demidov, who was orphaned and left in a miserable hellhole, eventually escaping and enlisting in the Russian army as a teen. When WWII came, he fought, and he eventually ended up in Berlin, where he become a propaganda icon thanks to his part in a pivotal battle and his posing for a famous photo. As the film gets going, Russians are adjusting to life in the '50s, and Leo works as an agent for the MGB.