If Prince Harry marries while his grandmother is still Queen but doesn't take on a peerage he and his wife will be TRH Prince and Princess Henry of Wales. Their children will be Lord X Mountbatten-Windsor and Lady Y Mountbatten-Windsor.
In this scenario, when Charles becomes King, Harry and his wife will become TRH The Prince and Princess Henry while their children will become HRH Prince X and HRH Princess Y.
If Prince Harry marries and is granted a peerage - say Duke of Clarence, Earl of Avondale, Baron of Athlone, then he and his wife will be TRH Duke and Duchess of Clarence during the Queen's reign, with their eldest son being Earl of Avondale, younger sons being Lord X Mountbatten-Windsor and daughters being Lady Y Mountbatten-Windsor.
In this scenario, when Charles becomes King Harry and and wife will become TRH The Duke and Duchess of Clarence, with all their children being either HRH Prince X of Clarence or HRH Princess Y of Clarence.