The actor hit the road with his pal Charlie Boorman for a motorcycle trek across 18 countries, from his native Scotland to the final destination of Cape Town, South Africa, as part of his documentary Long Way Down.
During his adventure, he went to Gulu in Northern Uganda, where he met children who had been forced to join rebel fighters, the Lords Resistance Army, and he admits their horrific tales still play on his mind years later.
In a column for Britain's The Independent, he writes, ""Over four years have gone by since that meeting. A lot has happened since then, but these memories remain absolutely vivid in my mind. Even now, it's difficult for me to accept or to get into my head what those children told me - because even after four years it's almost too awful to comprehend.
""These children were raped, brainwashed and forced to maim, torture and kill... For a child to be in any military situation is wrong. I think about my own children. Then I think about these children and as a father I imagine what it's like for their parents; suddenly their child disappears, they've no idea where they are and they're lucky if they see them alive again. They're just left to imagine the horrors that they might be going through.""
McGregor is urging fans to donate to Unicef, which provides aid for thousands of children and Ugandans displaced by fighting.