What happened to Lucy and Darnay's little boy?

Their son died.....

Even when there were sounds of sorrow among the rest, they were not harsh nor cruel. Even when golden hair, like her own, lay in a halo on a pillow round the worn face of a little boy, and he said, with a radiant smile, “Dear papa and mamma, I am very sorry to leave you both, and to leave my pretty sister; but I am called, and I must go!” those were not tears all of agony that wetted his young mother’s cheek, as the spirit departed from her embrace that had been entrusted to it. Suffer them and forbid them not. They see my Father’s face. O Father, blessed words!