Here is a source about François-Martial de Choiseul https://www.lemarois.com/jlm/data/j32choiseul.html (using several impeccable sources).
François-Martial de Choiseul, indeed "inspecteur général d'infanterie et menin de monsieur le Dauphin, capitaine des gardes du roi Stanislas, maréchal de camp, colonel du régument de Flandre, brigadier des armée, inspecteur général de l'infanterie , commandant des grenadiers de France" married Charlotte-Rosalie de Romanet in 1751 at Château de Bellevue (Madame de Pompadour's residence), and had one daughter, Charlotte-Rosalie (1753-1758). Charlotte-Rosalie de Romanet died in 1753, a few days after the birth of her daughter.
He married, secondly, Madeleine Thiroux, daughter of Pierre Thiroux de Montregard, comte de Médary, on 24 June 1755. They had no known children.
François-Martial died on 18 May 1791. His wife died on 12 March 1811.
I don't think it's likely that François-Martial and Madeleine suddenly had two sons nearly 20 years after their marriage and sent them on their own to Ontario (nothing in Le Marois's research indicates that they went to America. I assume they both died in France).
Concerning the Ontario side of the story, please read this well-researched profile, it's worth it. The man died on the same day and in the same place as your Francis Charles, and has known parents, from L'Ancienne-Lorette (Québec), and a father who wrote about his son who went away. The profile is well worth reading.