By Fergus Walsh
BBC News medical correspondent
As Ewan McGregor walked into the orphan care centre in Malawi the children began to sing their song of welcome.
Ewan McGregor went to Malawi with Unicef
They are some of the half a million youngsters who've lost one or both parents to Aids.
This was McGregor's first visit as Unicef ambassador - a five day packed schedule visiting hospitals, community projects and people's homes.
At the orphan affairs unit, we met scores of children whose lives had been turned upside down by Aids.
Yet with the education, finance and guidance they get here they are upbeat about the future, not downcast.
The centre helps look after younger siblings while they attend school.
We met a group of 50 teenagers who've formed their own parliament.
They have "MPs" in all the surrounding villages.
Together they discuss problems and provide support, counselling and sexual health advice. Many are HIV positive.
Children orphaned by Aids are encouraged to use theatre, dance, writing and painting to express how the disease has affected them.
But Ewan McGregor also saw how Aids is robbing children of their youth.
We met Salome, who's 12. Both her parents died of Aids-related diseases, so now she is sole carer for her four-year-old brother and her blind grandmother.
Ewan McGregor plans to use his trip to spread awareness back in Britain
And wherever we travelled in Malawi we came across similar stories.
Aids is wreaking havoc in Malawi. Some of the health statistics here are mind-boggling.
One in five children die before they are five. One in eight women die in childbirth. And average life expectancy has plummeted to just 39 years.
There are 70,000 children in Malawi who are HIV positive.
Almost all were infected at birth, the virus passing from their mother. Only now are anti-retroviral drugs becoming available here which can dramatically improve the chance of a baby staying free of the disease.
One mother, Esnort, who's 32 weeks pregnant, allowed us to witness her HIV test.
She waited 15 minutes and then the result came back. She has the virus.
At least now Esnort and her baby will receive the drugs which should keep them healthy.
And there's another crisis here. Malawi is facing its worst food shortage in over a decade.
Millions are dependant on food handouts. Hunger and HIV make a potent combination.
Both suppress the immune system - together they accelerate the onset of Aids-related disease.
Ewan McGregor was clearly deeply affected by all he saw. "I hope I can use my position to highlight the needs of the children here", he said.
"When I return to Britain I can give talks and help fund-raise for Unicef.
"I will know what I'm talking about because I've seen it with my own eyes.
"British people are very generous but it's very hard to keep such causes in the public eye. Just a small amount can make a big difference."
McGregor is planning a motorbike trip from John O'Groats to Cape Town and hopes to visit Unicef projects along the way including meeting again some of the remarkable children here.