The clip was posted on social media by Twitter user @HRHPWales, showing Kate crossing her fingers and saying “I hope so” to the royal fan.
The Princess added: “You never quite know with kids!” She then moved the subject to the weather for the Coronation service.
Judi James, a professional body language expert, spoke exclusively to Express.co.uk to explain how Kate handled the slightly awkward question on the royal walkabout.
She claimed: “Kate’s secret plan for keeping Louis in check over the Coronation was revealed in this clip to be a mix of optimism, good humour and keeping her fingers crossed.
READ MORE: Princess Beatrice’s significant weight loss after ‘upsetting’ pictures
“We’re so used to seeing Kate glide elegantly and calmly through royal events, like the proverbial swan through water.
“Therefore, it’s good to see that, like most parents, there’s a lot of furious paddling beneath the water line to make it all look so easy.”
Judi explained how the Princess handled her body language when she was asked about her five-year-old son.
“Kate’s body language is engaging, chatty and quite natural here and when she is asked about Louis she literally crossed both sets of fingers during her reply,” the body language expert continued.
“Her one raised brow when first asked about the big day is accompanied by an instant and instinctive raising of her hands to chest height and splaying the fingers out with the palms facing the person who is asking the questions.”
According to Judi, Kate displayed “mixed emotions” with her body language gestures.
She opined: “Her hand positions suggest mixed emotions: the left hand is held in a control gesture as though she is taking calm control of the day and any nerves.
“But the right hand is limper and more submissive, suggesting some worries about it all going smoothly, especially any bits that are out of her control.
“She then performs a small bounce on her heels and a smile to reassure her fans that overall she is looking forward to the event.”
Judi commented on how Kate reacted as soon as her five-year-old son was brought up in the conversation.
She opined: “When Louis is mentioned her expression of good humour remains in place despite what could have been a difficult question.
“Both hands come up in a ‘stop’ signal to suggest some playful concerns and it is at this point that we can see her literally crossing her fingers for good luck.”