In five years' time, the letters will be posted back to them. By then, one young person may hope to be well on the way to becoming the president of China. Another might want to be recruited as a spy for the FBI and there could be a budding Nobel Peace prize winner in the city.
Adults also took part in the "Dear Me" campaign by writing a letter about what has happened to the dreams they once had.
The campaign marked the fifth anniversary of Kids' Dream, the first child-led organisation promoting the rights of children in Hong Kong.
Its 150 members, most of whom are under 18 years of age, hope to report to the United Nations every five years on the condition of children in the city.
They also aim to find creative ways to educate the public about children's rights. The innovative "Dear Me" campaign, which was organised with the help of Against Child Abuse and the Hong Kong Committee on Children's Rights, is just one of them.
Althea Suen Hiu-nam, 15, is a member of Kids' Dream. She said the group was sent 1,500 "Dear Me" letters from adults and children. "The campaign helps you to remember your childhood dream," she says.
While there were many grand ambitions listed, the top three childhood dreams were to become a teacher, police officer or doctor.
Ring Huang Ying, 12, dreams of becoming a writer, and says China would be her subject. "I'm inspired by my father who loves the Chinese culture. And I, too, think we Chinese have a rich, interesting culture," she adds. "I also want to be a philanthropist. I think there's a wide gap between the rich and the poor in Hong Kong. Many people share a tiny room with many others in old buildings. I am concerned about the problem of poverty in the world."
Ring believes in the power of children's voices. "We can speak for our needs and promote our rights to participation, protection and development, as advocated under the United Nations Convention on the Rights of the Child," she says.
As for whether her dream can come true, she says: "I understand that I may encounter many difficulties. For example, if I want to be a philanthropist, I'll need a lot of money and it may be hard to find people who are willing to donate to charity.
"However, I will hold on tightly to my dream. And after five years, I might be amazed by what I have achieved."