Children with siblings and better educated parents tend to be less happy than their peers, a study by Lingnan University found.
The results surprised researchers, who said the survey suggested a worrying trend for young people to be more unhappy than adults.
"A surprising result is that better education on the part of the parents does not necessarily translate into happier families. This could be related to the stronger pressures on the children to live up to a higher expectation - their own or their parents," the report said.
Another surprise, the report says, is that brothers and sisters bring disharmony.
"When a family has more than one child, problems of inequality or parental troubles follow, leading to a less harmonious family life," said lead researcher Professor Ho Lok-sang, of the university's centre for public policy studies.
The report, commissioned by the Early Childhood Development Research Foundation, was based on a survey of 1,025 children aged eight to 17.
"Children's happiness index [a ranking of how happy they feel] keeps declining as they grow up and as they move up to more senior classes, falling from about eight [out of 10] at the age of eight, to below six at the age of 16," the report says.
"An average happiness score close to six is quite worrying, as it suggests a significant number of children have a happiness score of below six. These are much lower than the Hong Kong average of around seven in the adult surveys [in] recent years."
Children are also likely to be less happy if their parents are older - amid a growing tendency for Hongkongers to marry and have children later. "[But] the most important thing is parents have to let their kids speak, rather than telling them to 'shut up'," Ho said. He also questioned the government's lack of emphasis on personal development, saying it should be part of the curriculum at the city's schools.
"That's important as children have no life experience themselves. The only way they could learn how to handle different issues in their life would be by reading more stories about others and being guided by adults - their teachers.
"Or maybe the government thinks moral and national education matters more," Ho said, referring to the subject that drew tens of thousands of people to protests, amid claims it would be used for "brainwashing".