The couple spoke out as King Charles and Queen Camilla also shared their shock over the stabbing horror in Merseyside on Monday (29.07.24) - which also left six other youngsters and two adults who tried to protect them critically wounded.
Parents-of-three Catherine and William, both 42, posted on Instagram: "As parents we cannot begin to imagine what the families, friends and loved ones of those killed and injured in Southport today are going through".
"We send our love, thoughts and prayers to all those involved in this horrid and heinous attack".
"Thank you also to the emergency responders who despite being met with the most horrific scenes, demonstrated compassion and professionalism when your community needed you most".
Officers raced to answer reports of a knife attack at an address in Southport at 11.47am on Monday, where they found multiple victims had been subjected to a nightmarish attack that eyewitnesses described as straight out of a "horror film".
A 17-year-old boy from Banks, Lancashire, who is from Cardiff in Wales, has been arrested on suspicion of murder and attempted murder.
Merseyside Police Chief Constable Kennedy said: "It is understood that the children were attending a Taylor Swift event at a dance school when the offender, armed with a knife, walked into the premises and started to attack the children inside.
"We believe that the adults who were injured were bravely trying to protect the children who were being attacked".
A motive for the attack was not clear hours after the carnage, but it is understood it is not being treated as a terrorist incident.
Police say they are not looking for anyone else in connection with the stabbing and multiple witnesses described seeing a man in a black hood at the scene of the carnage.