William will take part in van Straubenzee's wedding festivities and serve as usher for the ceremony, while George and Charlotte are expected to join the wedding party as pageboy and bridesmaid, the outlet reports.
According to the Sunday Times, the godfather to 4-year-old Charlotte proposed to Lucy Lanigan-O'Keeffe, an assistant head teacher at Thomas' Battersea School in London, where George, who turns 6 on July 22, is now enrolled.
Mr van Straubenzee, 36, who is the boss at the private office of property firm Knight Frank, married the Duke of Northumberland's daughter Lady Melissa Percy in 2013. Princes William and Harry were among the guests.
It is unclear who introduced the couple past year but William and Kate are alleged to have played a part.
Lady Melissa, or "Missy" to her friends, moved to NY to live with hedge fund boss Remy Trafelet.
Ms Lanigan-O'Keeffe is originally from Kilkenny and is a sister of Olympic pentathlete Arther Lanigan-O'Keeffe and daughter of barrister Stephen Lanigan-O'Keeffe.
The van Straubenzee family has long had close ties to the royals.
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(Outdoor learning sounds like P.E., but is mindfulness being taught to kindergartners?) Von Straubenzee is also known as Prince William's longtime friend; he and his younger brother both attended school with William and Harry.
William and Harry are patrons of the Henry van Straubenzee Memorial Fund set up by his parents.
It has even been reported that he may become baby Archie's godfather when he is christened in July.
The Queen is also known to be fond of Thomas, a Balmoral regular.
Charlie and Thomas both went to Ludgrove School in Berkshire with the Princes, and were ushers at the Duke and Duchess of Sussex's wedding last May.
Princess Charlotte, four, will start at Thomas's Battersea in September.