The British actress travelled to Ethiopia to see the work that charity Save The Children does with teenage mums, some of whom became mothers at the age of 10.
Speaking to Hello! magazine, she said: "I felt honoured so many girls were prepared to share their stories with me, a stranger. What I found most difficult was talking to girls aged only 9 and 10 about marriage. Sex education is totally taboo.
"Although the legal age of marriage is 18, it's virtually impossible to implement it as birth certificates are rarely available and birthdays aren't celebrated. Meeting so many girls who had been promised to older men, seeing the fear in their eyes and how helpless they felt, has stayed with me."
Richardson believes that we need to look after all children of the world and that everyone can do something to help.
She added: "My time in Ethiopia was full of hope. I saw real change taking place, albeit slowly".