The 31-year-old met staff, children and their families at the Shooting Star Children's Hospice in Hampton near London.
The Duchess, a keen supporter of children's hospices and palliative care, toured the facilities, watched a music therapy session, met volunteers and had tea with the children and their families.
Kate had a private meeting with parents who recently lost their two young children to spinal muscular atrophy. Their one-year-old daughter died three days previously while they lost their five-year-old son to the disease seven weeks ago.
The family had come to the hopsice to spend time in the Tranquil Suite, a space dedicated for relatives to grieve.
In contrast to her dazzling red carpet appearance at the Mandela: Long Walk to Freedom premiere on Thursday night where she and William learnt of the death of Nelson Mandela, Kate opted for a more casual daytime chic look.
The working mum showed off her slender post-pregnancy figure in a fitted dove grey coat by one of her favourite high street labels Reiss.
She wore the £275 Delaney coat with dark blue skinny jeans and black suede wedge boots from Russell and Bromley.
The popular royal is no stranger to visiting children's hospices. As Royal Patron of the East Anglia Children's Hospices (EACH) she is involved in the extended hospice network around the UK.
"Children's hospices provide lifelines to families at a time of unimaginable pain," Kate said during Hospice Week last year. "The support they give is vital."
On 29 April, her second wedding anniversary, she visited Naomi House hospice when she was around six months pregnant with Prince George.
The hard-working royal gave her first ever public address at an Ipswich children's hospice in March 2012.
Her calm, self-penned speech, which lasted three minutes, hit the headlines. She spent the morning at the Treehouse hospice, which is part of EACH, meeting children, their families, staff and volunteers.
While Kate toured the hospice, her husband Prince William was in Aldershot presenting medals to soldiers who have recently returned from Afghanistan.
"You did your job magnificently, helping the people of Helmand to face their future with greater security and confidence, and you all came home safe and sound," the Duke told the First Battalion Irish Guards at Mons Barracks.
"This speaks greatly of your professionalism and to your care for one another.
"We are extremely proud of you, and glad beyond words that you are safely home."
The 31-year-old ex-RAF helicopter pilot briefly met some of the servicemen and their families.
William and Kate's schedule has become busier of late as they take on more royal engagements since the birth of their baby son and William leaving the RAF.