A patron of the charity, Kate, 37, officially opened the new Centre of Excellence at the Anna Freud Centre before meeting with children and their parents during her visit.
The mother-of-three's recent outing comes as the world awaits news of the arrival of Prince Harry and Meghan Markle's first born who is due any moment now.
This isn't the first official engagement Kate has attended this week.
After attending an Anzac Day commemoration service with her brother-in-law, the Duchess took part in two unannounced engagements, visiting Sayers Croft Wildlife Garden and Forest School at Paddington and then hosting a reception for the members of her Early Years Steering group at Kensington Palace.
Kate was also recently granted the title of Dame Grand Cross of the Royal Victoria Order (GCVO).
Her Majesty the Queen presented it to her granddaughter-in-law to mark her eight year wedding anniversary to Prince William on April 29.