HRH Prince/ss A of Cambridge I imagine. They are the heirs apparent, so definately HRH, yes? Should Charles live to inherit, or for that matter should C predecease QEII, William becomes POW... I dont know if the children's title changes to HRH Prince/ss A of Wales at that point?
Under the present LPs only the eldest son would be HRH.
The LPs currently restrict the HRH title to the children of the monarch - Charles, Anne, Andrew and Edward, male-line grandchildren of the monarch - William, Harry, Beatrice, Eugenie, Louise (I know she isn't using HRH but she is entitled), Duke of Gloucester, Duke of Kent, Prince Michael of Kent and Princess Alexandra of Kent (these last four are male-line grandchildren of George V just like the Queen and Princess Margaret who were also born HRH due to being male-line grandchildren). The spouses of the males also hold the HRH - thus Camilla, Sophie, Duchess of Gloucester, Duchess of Kent and Princess Michael of Kent are also HRH.
After male-line grandchildren the only other person to have the HRH is the eldest son of the eldest son of the heir to the throne i.e. William's eldest son would get HRH but none of his other children unless new Letters Patent are issued.
As for the Prince of Wales title William will NOT automatically get that title. It will be up to Charles assuming that Charles succeeds his mother.
When the present Queen dies, Charles will automatically become King and William will automatically become Duke of Cornwall and Duke of Rothsay etc. At some time after his accession Charles may decide to create William Prince of Wales. Charles himself had to wait 6 years after becoming heir to the throne. George V waited nine months or so after the death of Queen Victoria before Edward VII created him Prince of Wales. George V was known throughout most of 1901 as Duke of Cornwall and York.
If Charles should predecease the Queen she may decide to create William Prince of Wales but William then won't become either Duke of Cornwall or Rothsay as those titles can only be held by the eldest son of the monarch who is also heir to the throne.