Smocking is a classic embroidery technique, which is used to gather fabric so that it can stretch, which was used before elastic, and is commonly used in cuffs, bodices and necklines.
Speaking to Harper’s Bazaar, Marlen said the decision is more than just personal preference, because ensures they maintain a “clean, traditional look”.
“If you look at photos of young royal girls - from Princess Anne to Charlotte, you will notice that they tend to wear smocked dresses as little girls when they are in public with their parents," Marlen said.
“There are no rules about no pants. If the Duchess is in a dress in public and Charlotte is with her, most likely Charlotte, at this age, will be in a dress,” she added.
Similarly, Marlen said Kate tends to opt for a classic polo shirt and shorts when choosing George’s attire, which is a traditional outfit worn by young male royals.
She went on to say while Kate’s dress rules applies to official events and outings, Charlotte and George most likely get to wear more comfortable clothes at home.
Designer Rachel Riley, who designs clothing Kate has repeatedly used for her children, told the publication the Duchess is aiming to recreate a "timeless" look.
“The reason why I design clothes that way is because if they wear very simple things, it’s about the child, and it’s timeless in that you can’t really date a specific photo or put them in something that seems out of date,” Rachael said.