So according to this say William has a second son named Clamcy. He will not be an HRH at birth and then all of a sudden when his father accedes (and say Clamcy is 35 has a family of his own and has been sucessfully emplyed as a circus clown for years) he suddenly becomes HRH Prince Clamcy of the United Kingdom and has to give up his career to perform royal duties! That does not sound right.
That is exactly the situation under the current Letters Patent.
If either or both of William and Harry marry and have children during the Queen's reign only William's eldest son will be HRH from birth. Any other child/ren of William and/or Harry would get the HRH only when Charles becomes King. Of course if Charles dies before his mother then any children of Harry's won't get the HRH at all as they would never be the male line grandchildren of the monarch.
The current LPs give the HRH to
a) the children of the monarch - Charles, Andrew, Edward and Anne
b) male line grandchildren of a monarch - William, Harry, Beatrice, Eugenie, James, Louise (who aren't using it), Richard Duke of Gloucester, Edward Duke of Kent, Prince Michael of Kent, Princess Alexandra of Kent.
c) the eldest son of the eldest son of the Prince of Wales (namely William's eldest son only).
The fact that two of those entitled to the HRH are currently not using it suggests to me that in future the LPs might be changed to limit the HRH to the children of the monarch and only the eldest son/child of the heir to the throne. That would mean that people like Harry, Beatrice, Eugenie, James, Louise, Richard, Edward of Kent, Michael and Alexandra wouldn't get it in the future but I don't think that Charles would necessarily remove it from those who already have and use it (although they might voluntarily give it up).