The mother of a child with Down's syndrome said they will be walking towards a 300-mile challenge to fundraise for their "lifeline" charity.
Zephy Ojar, three, has support "he wouldn't otherwise have" from charity Down's South London (DSL), his mother Sarah said.
But the charity was "greatly impacted" by Covid-19 as it saw a £50,000 shortfall in donations.
It has organised a fundraiser to keep running its "vital" services.
The virtual Down's South London Kids Walk starts on Saturday and will see 15 children walk, run and cycle 300 miles between them in a month.
Ms Ojar, 37 and from Lambeth, said the charity has been helping since her son was a few months old.
"When you have a child with Down's syndrome and they're first born, you just don't know what life is going to be like for them.
"So when you have charity giving both the child and parents confidence, it's just something you can't value the measure of," she told the BBC.
Six-year-old Theodora Pyke, who is also supported by DSL, said she is excited to take part in the challenge as it will help her friends.
"I love therapy because it really helps me with my reading and my writing and my numbers and my talking," she added.
DSL has adapted to the restrictions imposed during the pandemic by providing therapy over Zoom and moving to online fundraising.
Its chief, Gün Akyuz, said it is "so proud" of the youngsters taking part in the challenge.
"We will continue to do whatever we can to support our fantastic community during this particularly tough time and to try to make up for the fundraising that's been lost so that we don't have to limit the support we can provide or worse, have to stop providing support altogether," she said.