The charge was treason. It happened on December 11, 1792. Sir Walter Scott covered it, including this poignant scene, in "The Life of Napoleon Buonaparte".
'It was the cruel resolution of his jailors to take the boy from his father on the very morning [December 11] when Louis was to undergo an interrogatory before the Convention. In other words, to give the deepest blow to his feelings, at the very moment when it was necessary he should combine his whole mental powers for defending his life against his subtle and powerful enemies.
This cruel measure produced in some respect the effect desired. The King testified more deep affliction than he had yet manifested. The child was playing at the game called Siam with his father, and by no effort could the dauphin get beyond the number sixteen, " That is a very unlucky number," said the child. This petty omen seemed soon accomplished by the commissioners of the Assembly, who, without deigning further explanation than that Louis must prepare to receive the Mayor of Paris, tore the child from his father, and left him to his sorrow. In about two hours, during which the trampling of many horses was heard, and a formidable body of troops with artillery were drawn up around the prison, the mayor appeared, a man called Chambon, weak and illiterate, the willing tool of the ferocious Commune in which he presided...'