Royal Titles and Succession
Charles: Prince of Wales
Camilla: Princess of Wales but she doesn't use it due to the sensitivity of her involvement with Charles during marriage to Diana. She uses Duchess of Cornwall.
William: Prince William. After marriage William is expected to accept a Dukedom. Duke of Clarence, Duke of Cambridge, Duke of Connaught or the Duke of Sussex are all possibilities. It is possible that William will not accept Dukedom and remain Prince William.
Kate: Is a Queen-in-waiting. After marriage she would become Her Royal Highness Princess William of Wales or if the Queen chooses, Her Royal Highness Princess Catherine of Wales. Shortening her title to Princess Catherine is not technically correct. The Princess title is legally reserved for princesses of birth. Upon William's Dukedom, Kate would become a Duchess. If William refuses Dukedom, Kate will remain Princess William of Wales or Princess Catherine of Wales.
Harry: Prince Harry
IF PRINCE CHARLES TAKES THE THRONE
Charles: Will become King Charles of England
Camilla: It's likely that she would take the title of Princess Consort, instead of Queen, to avoid public scrutiny.
William: Is bumped up to Prince of Wales.
Kate: Becomes Princess of Wales and Duchess of Cornwall.
IF CHARLES PASSES AND WILLIAM TAKES THRONE
William: Becomes King William V of England.
Kate: Becomes Queen Consort. "Consort" being the official title of the reigning monarch's spouse and would be known as Queen Catherine, though she would not be in line to rule if anything happened to her husband. Instead, the throne would pass directly to one of her children, likely the oldest male, or to Prince Harry, William's younger brother.