The three-year-old prince will be a page boy and his two-year-old sister will be one of the bridesmaids for their Aunt Pippa, the sister of Kate Middleton.
The children of Prince William and Kate, the Duke and Duchess of Cambridge, were expected to play a key role at the wedding of 33-year-old Pippa to her financier fiance James Matthews on May 20 and their actual roles were confirmed by Kensington Palace yesterday.
Prince Harry will also attend the wedding, leading to speculation if his Hollywood actress girlfriend, Meghan Markle, will be his plus one at the wedding to be held at St. Mark's Church in Englefield, Berkshire.
Pippa had confirmed her engagement to Matthews in July last year.
Matthews, 41, who works at a hedge fund had been dating for about a year and were living together.