The 101 Dalmatians star visited villages in Ethiopia to see the work British charity Save The Children is doing with teen mothers in the impoverished country.
She met several young girls, some who became parents as young as 10 years old, while others were married off to older men by their families.
But despite the harrowing stories she encountered, Richardson has hailed the trip as life-changing and is urging British Prime Minister David Cameron to pledge his support to help children across the continent.
She tells Hello! magazine, "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 nine 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.
"We need to look after children globally. We can all do out bit to put this right. My time in Ethiopia was full of hope. I saw real change taking place, albeit slowly."