One marcher, Shanell, explains: "The death of Damilola Taylor brings home the reality of racism. Young people are really worried about race attacks and about people living and dying in poverty."
Those on the "Rights Now!" march, organised by youth groups, want children's rights to be taught at school.
It also celebrates the 10th anniversary of the UK's agreement on the UN Convention on the Rights of the Child, which applies to under 18s.
They feel not enough's being done to stop bullying, poverty, and racism which affect young people.
And they hope the march to Downing Street, where they'll present Tony Blair with a birthday cake, will put pressure on the government to create a Children's Rights Commissioner for England to give young people a voice.
Andy, 14, explains: "There isn't anyone in the country who is just there for us. We need someone who can stand up for children and young people when the government is making laws and important decisions that affect us."