Princess Kate and Prince William have condemned the "heinous" Southport stabbings, adding that they "cannot imagine what the families went through".
A 17-year-old boy from Banks in Lancashire remains in police custody following the stabbings - but the incident is not being treated as terror-related.
The royal couple said in a statement: "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.
Princess Kate and Prince William 'cannot imagine what the families went through' following Southport stabbingsPA/Getty"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."
The message was signed off personally with their initials "W & C".
Meanwhile, the King issued his own statement on the attack, which he described as "utterly horrific".
"My wife and I have been profoundly shocked to hear of the utterly horrific incident in Southport today," he said earlier today.
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Police at the scene in SouthportX/@ChauffeurWest
Forensic investigation working at the scene in SouthportPA"We send our most heartfelt condolences, prayers and deepest sympathies to the families and loved ones of those who have so tragically lost their lives, and to all those affected by this appalling attack."
Six of the children injured are in a critical condition. Two adults are also in critical condition, who police believe were "bravely trying to protect the children who were being attacked".
The children are understood to have been taking part in a sold-out Taylor Swift-themed class at a dance centre in the town when the attack was launched.
Officers were called to reports of a knife attack on Hart Street in Southport at 11.47am.
The couple said: 'We send our love, thoughts and prayers to all those involved in this horrid and heinous attack'
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Merseyside Police Chief Constable Serena Kennedy speaking to the media at Southport Community Fire CentrePAChief Constable Kennedy said that when they arrived, they were "shocked to find" several children had been "subjected to a ferocious attack and had suffered serious injuries".
Armed police officers then detained the 17-year-old and seized the knife.
Colin Parry, owner of Masters Vehicle Body Repairs, also on Hart Street, said earlier today he believed that six or seven "young girls" had been stabbed.
Another witness at the scene said he called police to an address behind the Hart Space studios, where a yoga class for children in school years two to six - aged between six and 11 - was underway.
Merseyside's Police and Crime Commissioner Emily Spurrell said she was "utterly shocked" by what happened, adding: "These young people had their whole lives ahead of them and it is hard to find the words to convey our deep sadness."
