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 likely or unlikely sources.
From Nostradamus, a most reliable source, 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 following letter is published by kind permission of the Daily Mail; it was published on October 21, 2010, in the printed version of the Daily Mail and is not available online; I therefore include it in full length.)
“Further to the earlier answer, ever since my great, great grandfather (Karl Wilhelm Naundorff) died in Delft, Holland and was buried Louis XVII and a national monument erected the French government have made strong objections, but to no avail.
It had been very difficult to keep the whole story alive, since my opponents came up with the phony heart and declared it to be from Louis XVII. The persons who instigated this DNA research have tried to oppose me and my family for over 200 years. They are supporters of the Bourbon Parma family who now claim to be the sole inheritors of the name. One key supporter is the Duc de Beaufermont, official guardian of the Tombs in the church of St Denis where all the royal family are buried. They therefore have lots of hearts to choose from. However that does not matter since the heart Dr. Pelleton took from the so-called Louis when he died was always the heart of the substitute boy that died in the Temple. His body lies in the church of St. Marguerite and was declared to be ‘Not Louise XVII’. We tried to get that body through a DNA research test but French government declared that it would not give permission since ‘it was known the body was Not Louis XVII’. We wanted to match the heart but it was not allowed. No wonder, it was also false. A study exists on the internet at a site called ‘musée Louis XVII’ and a film was made pointing out the errors called ‘Querelle sue un Throne’ by Jean-Charles Denian and shown on French television.
When my forefather applied to receive his name back, he was exiled to England. My Great Grandfather Adelberth was born in England. His birth was registered on May 02, 1840 at Camberwell, Surrey, England, showing his date of birth as April 28, 1840 at Camberwell Green. His name is given Adelberth (prince of France) to His Royal Highness Charles Louis Duc du Normandie and Johanna Duchess of Normandie formerly Madam Einert.
For over 200 years we have fought to get our name recognized in France but we are fighting the republic which greatly fears a return of the Monarchy and also of course the branches Orleans (in charge while above suit was filed) and the Parma branch which now claims rights to the French Throne. And they keep coming up with new research like the DNA research and books supported by their group. The heart, if it came from Dr. Pelleton, who lost it, came from a body that could not possibly be that of Louis XVII - it was too large, too old etc.
The previous answer states that the DNA of the heart was comparable with the DNA of Marie Antoinette. This is not true, but it should have been. The doctors say very distinctly the DNA matches with the two cousins of the Hapsburg family and therefore linked to Marie Antoinette. This is not the same thing. They also say that this is not proof that this heart was that of Louis XVII.