By RACHEL DODES

In the French film “Declaration of War,” a star-crossed couple named Romeo and Juliette huddle together in a hospital bed before their 18-month-old son is scheduled to undergo brain surgery. Trying to calm their minds, they say their fears aloud, laughing as they speculate that their baby may be rendered paralyzed, deaf, blind, a dwarf, black, or—quelle horreur!—gay. Soon after, they break into song.

“There’s almost like a social rule that says in this situation you have to be miserable,” says actor Jérémie Elkaïm, who co-wrote the film with director Valérie Donzelli. “But, in fact, you continue to live, almost more intensely.”

“Declaration of War,” opening in the U.S. on Jan. 27, is the latest film that attempts to explore the humorous aspects of one of the least funny circumstances known to man: the illness or death of a loved one. It follows other recently released tragicomedies such as “50/50,” about a young man (Joseph Gordon-Levitt) struggling with dating and a spinal tumor, and “The Descendants.” which focuses on a father (George Clooney) who discovers he’s been cuckolded while his wife lays in a vegetative state.

Watch a clip of the French tragi-comic film “Declaration of War” in which a young couple wrestle with their child’s cancer diagnosis. Video courtesy of IFC Films.

The more realistic approach to life-or-death situations reflects “a different generation, and a different generation’s approach,” says Jonathan Sehring, president of Sundance Selects, the film’s U.S. distributor. “This isn’t the Hollywood version.”

Shot for about $1.7 million using a Canon 5D camera and a tiny eight-person crew, “Declaration of War” became an unexpected hit in France following the film’s debut at the Cannes Film Festival, with nearly one million admissions, making it the most successful low-budget film there in a decade.

The film was selected as France’s official submission to the Academy Awards in the foreign-language category—a highly unusual feat for an independent film featuring actors that aren’t well known outside of indie circles. But on Wednesday, judging committees unexpectedly cut it when they winnowed the entries down to nine. France is accustomed to being on the short list in this category, and this added to the indignity of the loss last year by its critically acclaimed submission, “Of Gods and Men.”

“France is the only country where it is a major scandal if one of their movies doesn’t get nominated,” says Mr. Sehring, who timed the U.S. release to coincide with the Academy’s announcement of the nominees.

Like “50/50,” which was loosely based on screenwriter’s own bout with cancer, “Declaration of War” was inspired by actual events. Ms. Donzelli and Mr. Elkaïm, who co-star as Juliette and Romeo, found out that their son, Gabriel, had a brain tumor in 2003. After five years of intense treatment, he was declared cancer-free.

Ms. Donzelli and Mr. Elkaïm began talking about using their harrowing experience as way to tell a love story. “We didn’t want the audience to be hostage to a film about pain and tragedy,” says Ms. Donzelli. She who infused the film with spirited dance sequences, original songs and action shots, mostly of the couple cruising around Paris on a scooter.

There are some documentary-like aspects to the film. The hospital scenes were shot at the Gustave Roussy Institute outside of Paris, where Gabriel was treated, and the screenwriters decided to name the surgeon Professor Sainte-Rose, as a tribute to the doctor who actually operated on their son. (Ms. Donzelli asked the doctor to play himself, but he declined, saying that acting wasn’t his forte.) Gabriel, to whom the film is dedicated, appears at the end of the film as an older version of the baby, named Adam in the movie. And like the fictionalized characters Romeo and Juliette, who were ultimately driven apart by the intensity of their situation, Ms. Donzelli and Mr. Elkaïm are no longer a couple romantically, but they still are close collaborators on films. Mr. Elkaïm will star in Ms. Donzelli’s next feature “Main Dans la Main” (“Hand in Hand”) which they were shooting in New York this month.

Write to Rachel Dodes at rachel.dodes@wsj.com