Love hearts on the dress were seen to include messages such as 'Choke', 'Please Drown', 'Drop Dead', 'Drink Poison' and 'Eat S**t'.
Frost wrote on Twitter: "I seem to have upset people & am shocked myself about the dress Iris wore to Vivienne Westwoods show. Iris had been bought it d as a present. (sic)
"I assumed it was just the sayings the sweets have on them. I'm sorry for any upset it may have caused..Iris is a sweet , innocent girl."
Describing it as a "genuine mistake," Frost added: "We were enjoying a day out with my mum me @ iris and it was a lovey day.I guess the dress should be hidden till she's older."
Jonathan Rhys Meyers, Janice Dickinson, Jo Wood, Christina Hendricks, Kimberly Wyatt, Rita Ora and Amber Atherton were among the other guests at the Westwood show.
> See more pictures of the celebrity guests at London Fashion Week