 -- Prince William and Princess Kate joined Queen Elizabeth and Prince Philip today for the first of three annual garden parties at Buckingham Palace.

While speaking with guests, Kate, 35, revealed one concern about her sister Pippa Middleton's wedding to financier James Matthews this weekend.

Kate, the mother of Prince George, nearly 4, and Princess Charlotte, 2, admitted she was excited for the wedding, but nervous about her children's roles as bridesmaid and page boy.

"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, from Coventry, told reporters. “She said she was hopeful that they would be good but you never know at that age.”

The garden parties have been a tradition since the 1800s, with the coveted invitation a reward for public service. Approximately 8,000 guests line up each year to get a glimpse of the royal family and hopefully have a word with the monarch and her family.

The Queen’s staff serves more than 27,000 cups of tea and guests consume 20,000 sandwiches and 20,000 slices of cake, according to Buckingham Palace. Formal dress is required. Men are often in top hats and morning suits while women where their best dresses accessorized with an appropriate hat or fascinator.

Kate donned an ice blue Christopher Kane coat and a matching hat from Lock & Co., which she also sported at the opening ceremony of the Olympic Games. Queen Elizabeth also recycled the pink dress and hat she wore to Princess Charlotte's christening.

While Kate and William attended the party, preparations are underway for Middleton's wedding on Saturday. A large glass tent structure, which is estimated to cost more than $100,000, has been erected at the Middletons' 18-acre Bucklebury manor house, where the reception will take place.

The wedding ceremony will be held in St. Marks Church on the private Englefield estate. The estate is the home of one of Britain’s wealthiest members of Parliament Richard Benyon, who has an estimated net worth of $150 million.

Workers have started erecting signs that public walkways will be closed on the grounds Saturday.

Stylists and fitters were spotted Tuesday bringing a number of garment bags to the $25 million townhouse Middleton shares with Matthews, prompting speculation that one may contain Middleton’s wedding dress and bridesmaid dresses.

Kate's hairdresser, Amanda Cook Tucker, was also seen entering the home. While Middleton and her mother, Carole Middleton, have frequently gone to celebrity hairdressers Richard Ward and James Pryce, some say Tucker could have a role styling Middleton or the bridal party.

Designer Giles Deacon, who is rumored to be designing Middleton’s dress, was also seen entering her home shortly after her engagement with a number of designs. Deacon has not confirmed the speculation.

The same guessing game took place when Kate married William in 2011 with Sarah Burton of Alexander McQueen ultimately doing the honors.

Nicki McFarlane, who designed the outfits for the little flower girls and page boys in William and Kate's wedding, has been tapped as the likely choice to make George and Charlotte’s outfits.

It has been a busy week for William and Kate in London ahead of Middleton's wedding. Prince William made an early morning visit to observe the work at the Royal Marsden Cancer Hospital before he and Kate joined the afternoon garden party at Buckingham Palace. Prince William is the president and patron of the hospital, a role held by Diana until her death in 1997.