Louis XVII would have been King of France after the beheading of his father Louis XVI but for the fact that France was a Republic at the time. He died while imprisoned in Paris; or maybe he didn’t. The case is puzzling to this day, and conflicting evidence is offered from any likely or unlikely source.
From Nostradamus we knew beforehand that the Royal children would survive their imprisonment by the leaders of the republic, while their parents would face the guillotine. While the survival of Princess Marie Thérèse was due to Austrian diplomacy, her brother Louis was declared dead in 1795 by the powers that be.
Many different stories about the escape of the young king can be found. If you really enjoy being confused, Wikipedia is the site for you. The stories in different languages diametrically contradict each other. Enough to say that there are different opinions; and there are many of those.
As to Royal descendants, I leave the explanation to one of them...
The Descendants of Louis Xvii of France | Socyberty
by Lucas Dié