Kensington Palace confirmed that George, 3, will be a pageboy in the wedding while Charlotte, 2, will be a bridesmaid. Their parents, Prince William and Princess Kate, will both attend the wedding Saturday but it is unclear if Kate will take on matron of honor duties in her sister's wedding.
Royal watchers predict it is more likely that Kate, 35, will be on the sidelines so that she does not upstage Middleton.
"She said that they were all really looking forward to her sister's wedding at the weekend but she was a bit worried about how her children might behave,” attendee Andrew Bates told reporters. “She said she was hopeful that they would be good but you never know at that age.”
Middleton, 33, and Matthews, 41, will tie the knot at St. Mark’s Church in Englefield, Berkshire, located just six miles from Bucklebury, where Middleton was raised.
While the American wedding tradition is to include bridesmaids closer in age to the bride, the British tradition varies.
Queen Elizabeth selected women close to her own age as bridesmaids in her 1947 wedding. Princess Diana used school-age girls as bridesmaids, rather than flower girls, at her 1981 wedding to Prince Charles and did not have a maid of honor. Queen Elizabeth's sister, Princess Margaret, also selected young attendants as bridesmaids at her 1960 wedding as did the queen's daughter-in-law, Sophie Wessex, in 1999.
Conversely, Autumn Phillips, the bride of Queen Elizabeth's grandson, Peter Philips, featured bridesmaids close to her age at her 2008 wedding. Kate's 2011 wedding to William included young bridesmaids led by Middleton as maid of honor.
Middleton led the bridal party down the aisle at Westminster Abbey holding the hands of the two youngest bridesmaids, Lady Grace van Cutsem and Lady Eliza Lopes, both then 3.
Kate's four young bridesmaids each wore ballerina-length dresses using the same fabric as Kate's wedding gown, adorned with a pale gold sash made of wild silk.
Nicki MacFarlane, who designed the bridesmaid and page boy outfits for William's and Kate's wedding, is rumored to be the leading contender to do the same for Middleton's wedding.
The details of Middleton’s wedding dress have not been revealed, although British designer Giles Deacon is the favorite. Deacon was seen bringing a number of garment bags earlier this year into the London townhouse Middleton shares with Matthews.
The role of the page boy in weddings has historically consisted of carrying the bride’s train and helping with ritual parts of the wedding ceremony. Page boys have remained a tradition mainly in royal weddings.
George’s father, Prince William, fulfilled the role at Prince Andrew's wedding to Sarah Ferguson in 1986, and a young Prince Harry served as a page boy at the wedding of Princess Diana's brother, Lord Charles Spencer, to Victoria Lockwood in 1989.
William and Kate chose as page boys for their wedding the young sons of members of the Buckingham Palace household, past and present, rather than fellow royals or sons of aristocratic friends. The two page boys, Billy Lowther-Pinkerton and Tim Pettifer, wore child-size uniforms in the style of Foot Guards officers worn at the time of the Regency (1820s).
The pair -- Lowther-Pinkerton is the son of William's and Kate's longtime equerry and private secretary and Pettifer is the son of William's and Harry's beloved childhood nanny -- made their way from the Clarence House, a royal residence, to Westminster Abbey in a series of escorted Bentleys.
George and Charlotte are Middleton's only niece and nephew. She and Kate also have a brother, James Middleton, who is not married.
The last time Charlotte and George were seen in public was in December attending church with their parents over Christmas in Berkshire, where the wedding will take place.