“She was with Charlotte and George looking at Halloween outfits but her bodyguard was kind of watching people with phones and telling them no pictures,” Ms Whittaker said.
"The lady that told me she was there had children with her and she said Kate was asking her children what they were going to be for Halloween.”
While she didn’t see the exact outfits the Duchess chose for her little ones, a royal insider previously told US Weekly Charlotte has a penchant for unicorns.
Another witness called Kathy said Kate looked “beautiful as always” and appeared “down to earth” as she blended into in with other shoppers at the supermarket.
“It was so lovely to see her, she is always beautiful. You don’t expect to see that when you go shopping. I couldn’t concentrate on my shopping,” Kathy said.
According to the bystander, the Duchess cut a casual figure in a black polo neck top and black trousers, while little Charlotte sported a checkered skirt.
Both shoppers praised the Duchess for being so "relatable", while other royals might have simply opted for more luxe stores such as Harrods or Harvey Nichols.
A children's witch outfit from Sainsbury's reportedly costs about $16, while a skeleton outfit costs approximately the same.