The 15-year-old, who cannot be named, discussed the potential attack in a far-right group he had created on the encrypted Telegram platform.
In September, he wrote: “I am planning an attack against the Dover coast where every Muslim and refugee has been given safety. If you’re interested tell me now.”
When another member asked what could be used in the attack, he listed potential weapons including Molotov cocktails and “metal bats”, while advising people to wear thick clothing that he claimed would protect against Tasers.
The boy, from Derbyshire, pleaded guilty to encouraging terrorism, and to possessing and disseminating a terrorist publication, on the first day of his trial on Monday.
Westminster Magistrates’ Court heard that he had a previous conviction for threatening to blow up a mosque on 20 January last year, in what chief magistrate Paul Goldspring said had been described to him as a “bomb hoax, a prank and a joke”.
He was handed a referral order on 25 September 2020 after pleading guilty to an offence under the Malicious Communications Act.
He appeared in court alongside a 16-year-old boy from southeast London, who was a member of the same online group and had written racial slurs, referring to people as “P***s”.
The older boy added: “It’s gotten to the point I will casually walk up to someone with a gun and ‘POW’.” He admitted dissemination of a terrorist publication.
Mr Goldspring adjourned sentencing for both boys to 3 August for reports to be prepared, but said: “The custody threshold has been crossed.”
The 15-year-old, who appeared by video link from a youth remand centre, was remanded in custody, while the older boy, who appeared in court with his mother, was granted conditional bail.
The teenagers were arrested at their family homes in a coordinated police operation on 22 September.
Prosecutors said that police analysis of their phones and devices had “found a large quantity of extreme right-wing propaganda”, including photos, videos and documents.
The younger boy had downloaded footage of the Christchurch terror attack – in which a gunman filmed himself shooting 51 Muslim worshippers dead at mosques in March 2020 – when he was 14 years old.
The following month, the older defendant made videos involving Adolf Hitler, Nazis shooting victims in concentration camps, and a woman singing: “All Jews should die, race mixing is a sin”. He was 15 at the time.
The same boy had made numerous internet searches relating to guns, weapons and bombs.
The younger defendant established the online Telegram channel they were prosecuted over in August last year, and set about trying to recruit members on Instagram, Facebook and Twitter.
He saved and shared the “Anarchist Cookbook 2000” document, which contains instructions on how to manufacture explosives.
The older boy was prosecuted for disseminating a different terrorist publication, which prosecutors described as a “guide to committing terrorism in the name of right-wing extremism”.
Both boys initially denied the offences. The 15-year-old allegedly laughed when he was arrested, and told police: “Basically I’m far right and you guys don’t like it.”
During police interviews, he denied writing the post about attacking Dover and said he had not created the Telegram group.
The court heard he had admitted the offences on the basis that his actions were “reckless” rather than “intended”.
The 16-year-old admitted being part of the group when he was arrested at his home, but said he had “decided it was a bit too much and left”.
It is the latest terror case to involve child defendants in the UK. Earlier this month, official figures indicated that more than one in 10 terror suspects arrested in Britain is now a child.
The senior national coordinator for counterterrorism policing, Dean Haydon, said officers are seeing concerns about increasing numbers of children being drawn into extremism “come to fruition”.
“Covid-19 has driven huge numbers of people to spend a lot more time online, and we have seen an increase in the volume of online extremism – much of which sits below a criminal threshold, but which creates a permissive environment which makes it easier for extremists to peddle their brand of hatred,” he added.
“We cannot hope to arrest our way out of this problem – the only way we can hope to reverse this worrying prevalence of children in our arrest statistics is to stop them from being radicalised in the first place.”
Additional reporting by PA