The full title of this wotk is "Fête donnée par le prince de Conti au Prince héréditaire Charles Guillaume Ferdinand de Brunswick-Lunebourg, sous la tente, en forêt de Cassan, à l'Isle-Adam en 1766." According to his Wikipedia article, the Prince of Conti was a spinster by the time this was painted. He had a falling out with his brother Louis XV, instigated by Madame de Pompadour, that led to his exile on l'Isle-Adam. He lost his wife Louise Diane d'Orléans in childbirth in 1736. His guest, Karl Wilhelm Ferdinand, was married to Princess Augusta, King George III's sister, in 1764.
Photo - Gérard Blot; image enlarged one half and exposure increased.