The teenager, who is from the nearby village of Banks in Lancashire, but was born in Cardiff, was arrested on Monday and a knife was seized by police.
A 17-year-old boy has been charged with murder and attempted murder after a mass stabbing in Southport.
Three children, six-year-old Bebe King, seven-year-old Elsie Dot Stancombe and nine-year-old Alice Dasilva Aguiar, died as a result of the attack at The Hart Space community centre.
A “Taylor Swift Yoga and Dance Workshop” aimed at children aged between primary school years two and six (seven to 11 years old) was taking place at the time of the attack in the seaside town, which is north of Liverpool.
The suspect, who is from the nearby village of Banks in Lancashire, but was born in Cardiff, Wales, was arrested on Monday.
He has been charged with three counts of murder, 10 counts of attempted murder and possession of a knife.
He will appear at Liverpool City Magistrates Court this morning.
Sarah Hammond, chief crown prosecutor for Mersey-Cheshire, said: “We remind all concerned that criminal proceedings against the defendant are active and that he has the right to a fair trial.
“It is extremely important that there should be no reporting, commentary, or sharing of information online which could in any way prejudice these proceedings.
“Our thoughts remain with the families of all of those affected by these harrowing events.”
Eight other children suffered stab wounds in the attack, and five of them remain in a critical condition.
Two adults – including one of the organisers of the event – are also in critical condition after being injured.
Riot police clash with protesters
More than 100 people were arrested in London on Wednesday night as a violent protest broke out in the wake of the Southport attack.
Flares were thrown towards the gates of Downing Street and some among the crowd threw cans and glass bottles.
Protesters later tried to kick down a fence as they were confronted by riot police.
Those arrested were detained for offences including violent disorder, assault on an emergency worker and breach of protest conditions, the Metropolitan Police said.
Some officers sustained minor injuries, it added.
Some among the crowd were heard shouting the name of far-right activist Tommy Robinson, while others shouted “Rule Britannia”, “save our kids” and “stop the boats”.
Protesters were penned in around the Cenotaph war memorial by officers attempting to control the disorder.
Police car set on fire in Hartlepool
Meanwhile, eight people have been arrested after protests broke out in Hartlepool on Wednesday night.
People have been detained for various offences after protesters threw missiles, glass bottles and eggs at officers, police said. Several officers sustained minor injuries as a result.