Louis' artwork was likely created in one of William and Kate's home-schooling lessons during the coronavirus lockdown, with the Duchess recently explaining she kept the make-shift classroom up and running during the Easter holidays.
Speaking to the BBC, the mum-of-three shared: "It's just having that bit of structure, actually. It's great, there are so many great tips online and fun activities that you can do with the children so it hasn't been all hardcore."
A keen photographer, Kate is known for taking her own photos of her kids on their birthdays and special occasions such as their first day of school.
Last December she revealed her youngest son had just started to talk.
Duchess Catherine couldn't help but compare a little boy she met at Peterley Manor Farm in England's Buckinghamshire - where she was helping children pick out Christmas trees for their classrooms - to her youngest son after the boy kept trying to get her attention by holding up his hand and saying, "Me, me".
The Duchess stroked the boy's cheek and told him: "You remind me of my little Louis, he keeps saying, 'Me, me, me.' And he wants to come everywhere with me!"