The youngster was among 38 people killed when the Lamma IV collided with a passenger ferry.
The cruiser, which was carrying partygoers to a fireworks show to celebrate China's national day, sank shortly after the crash while the ferry was able to limp to shore.
More than 100 people were taken to hospital after some were thrown into the water from the party boat.
The Foreign Office has confirmed that a British national was among the victims.
A spokesman said: "We can confirm the death of a British national in the Hong Kong ferry sinking. Consular officials from the British Consulate General in Hong Kong are in contact with the family and are providing consular assistance."
Hong Kong authorities said five children died in the crash.
The passengers on board the pleasure cruiser had gathered on the island of Lamma, around two miles south-west of Hong Kong island, to watch the fireworks display from the water, according to reports.
The majority of the 120 passengers were employees of Hong Kong Electric and their friends and family.
The crash is Hong Kong's deadliest accident in more than 15 years and its worst maritime accident in more than 40.