Theodora, 6, is the child of American actress Ayda Field and English singer-songrwiter Robbie Williams.
"A royal wedding is a private, personal event, dressed up as a national occasion", sniffed one protest petition signed by almost 40,000 people. Some even reminisce about the sunny afternoon in 2011 when Prince William married Kate Middleton, whose grace stirred memories of his mother, the late Princess Diana.
The 28-year-old princess, who is ninth in line to the throne, is due to marry liquor company executive Brooksbank Friday in St. George's Chapel at Windsor Castle.
The venue is where Harry, younger son of heir-to-the-throne Prince Charles, Wednesday USA actress Meghan Markle at a glittering ceremony in May and Eugenie is copying some of her cousin's plans for her big day.
"Although Princess Eugenie's engagement ring is reminiscent of Catherine's with the large centre sapphire and diamond surround, it's a classic design for a royal engagement ring and not likely an intentional similarity", royal commentator Eloise Parker said.
Prince Edward's children, 14-year-old Lady Louise Windsor and 10-year-old Viscount Severn will be special attendants.
The carriage will process through the grounds of Windsor Castle, departing via Castle Hill to proceed along part of the High Street before returning to the Castle via Cambridge Gate.
The Queen will host a wedding reception at Windsor Castle for the couple and their guests.
And while Princess Eugenie and Jack Brooksbank wedding is set to be a totally different event (it's a two-day affair with a funfair on Saturday), we'd say the royal family are getting pretty excited.