Wildlife expert Chris Packham led more than 100 children to deliver a petition to Buckingham Palace, calling on the royal family to re-wild their estates.
Accompanied by parents, the campaigners carried the Wild Card campaign petition – signed by 100,000 people – from Green Park tube station in central London to the palace.
Many of the children and their parents wore headdresses of leaves and flowers as they marched waved banners to the sound of a brass band.
Speaking outside the palace, TV Presenter Packham said: “Frankly, my conscience is not clear.
“On my watch as an environmentalist and conservationist, I have failed these young people – I have failed to act quickly and broadly enough to prevent the crisis that we find ourselves in.
“The world that they are likely to inherit – unless we act urgently, properly and quickly now – will not be as pleasant as the one I inherited when I was their age. And that’s not something I want to take to my grave.
“It’s the most harmonious, beautiful and peaceful demonstration I have been on for some time.
“We’re asking our royal family, denizens of the property behind us, to rewild the 800,000 acres of land that they have in the UK.
“If they were to do so it would be a very powerful message that would resonate with people all over the world.”
The choir SOS From The Kids, who featured on Britain’s Got Talent in 2019, also joined the march and performed. The choir will also perform at Cop26.
A spokesperson for the Royal Estates said: "Members of the Royal Family have a longstanding commitment to conservation and biodiversity, and for over fifty years have championed the preservation and development of natural ecosystems.
"The Royal Estates are constantly evolving and looking for new ways to continue improving biodiversity, conservation and public access to green spaces, as well as being home to thriving communities and businesses which form part of the fabric of the local community."
Noah Macaulay, 16, a sixth-form student from Hampshire and co-founder of SOS from the Kids, said: “The royal family has an amazing opportunity, because they own so much land, to lead by example and rewild that.
“They could really, really help to improve nature and help biodiversity.”
One of the petitioners, Hannah Clayton, 41, from Islington, north London, attended the march with her young son.
She said: “The royal family have a huge amount land, they are really committed environmentalists, and I think rewilding would be a really easy thing to do and makes such a big difference to biodiversity.”
Barbara Cope, 70, from Hampshire, whose granddaughter is a member of SOS From The Kids, said: “I think it’s so wonderful the children are here to support rewilding and being aware of the environment. I think people will listen to children.”
The royal family is the UK’s biggest landowner with an estate that includes the Crown Estate and the Duchies of Lancaster and Cornwall.
Campaign group Wild Card says the family owns 1.4% of the land of the UK.
They say that while the average tree coverage is 37% in the European Union, the Duchy of Cornwall estate owned by the Prince of Wales has only 6% tree coverage.
Ecologists believe the royal estates would naturally feature beavers, wolves, bison, wild boar, pine martens and white storks and these creatures could be introduced if re-wilded.
The Wild Card campaign is also demanding that 50% of land in the UK be fully re-wilded.
The Queen and senior members of the royal family will attend events during the Cop26 UN Climate Change conference, Buckingham Palace said.
The summit in Glasgow will run from October 31 to November 12.