Middleton donned a blue polka-dotted dress with a white collar, holding her son as Charles gazes fondly at Louis while holding his hand.
Little sister Princess Charlotte followed suit on May 2, 2015.
Prince William, who has been the royal patron of Tusk since 2005, wore a tuxedo and placed a poppy on his lapel in honour of Remembrance Day and the upcoming centenary of Armistice.
But William also says he wishes that his dad would slow down enough to be able to spare more time for them.
Manchester United's Paul Pogba out of Manchester City clash with injury
But City's scouts have so far failed to find a player capable of replacing one of the most important cogs in Guardiola's team. You can not expect to win three away games like we had this week (also beat Bournemouth last weekend).
Stepping out last night in a teal floor-length lace gown by British designer Jenny Peckham for Tusk Conservation Awards at Banqueting House in London, Middleton looked radiant but people couldn't help but get a sense of deja vu.
"It's something I'm working more heavily on, put it that way", William, 36, says in the documentary.
Charles's wife, Camilla, Duchess of Cornwall, 71, told the program that he is great with her grandchildren.
According to Camilla's childhood friend Broderick Munro-Wilson: "She was into boys much quicker than other girls her age".
Charles is about to become a grandfather for the fourth time when Prince Harry and Meghan Markle welcome their first child in the spring.