Time Out says
If Daniel Radcliffe wanted to prove himself in a role other than Harry Potter, he might have considered a better choice than My Boy Jack. Although he does strong, introspective work, everyone else in the film is elbowed out of the frame by David Haig’s oversize performance as literary lion Rudyard Kipling. As World War I draws close, the author’s only son, Jack (Radcliffe), desperately tries to pass a military physical despite severe nearsightedness. Jack does so with the glum resignation of someone reluctantly playing a role: His father has been beating the drum for war with Germany and calling on the British people to ostracize young men who refuse to volunteer.
Haig also wrote the screenplay. The hamminess of his performance suits the material, allowing him to portray Kipling as a man with a strong sense of his own importance, and one who doesn’t shy from the perks of success (such as friendship with King George V). The flip side to this, of course, is obligation, and Kipling’s swagger turns to self-flagellation as he realizes the world will brand him a hypocrite if he doesn’t essentially offer Jack as a human sacrifice. As the author’s outspoken American-born wife, Kim Cattrall almost musters the force required to match Haig, and when she comes close, Jack transcends its particulars and becomes a timeless indictment of militarism.