Duchess Catherine took Prince George to see the dinosaur gallery at the Natural History Museum in South Kensington last week.
The royal and her two-year-old son were spotted mingling among other visitors at the London museum as they checked out the exhibit.
Fellow visitor Rimi Rahman told PEOPLE: "She was holding George's hand, showing him around and talking to one of the people who worked there.
"People didn't really notice her at all. They went all around the dinosaur gallery - George looked so cute and little, his hair is so nice. He looked so happy."
A museum spokesperson said: "We were very happy to welcome Natural History Museum Patron HRH the Duchess of Cambridge for a private visit."
Catherine was casually dressed in black-and-white print capri pants with black ballet flats for her low-key day out with her son.
There was no sign of Prince William or their daughter Charlotte, who was born in May.
The museum is only a short drive from Prince William and Duchess Catherine's home at Kensington Palace.