The London 2012 star was at Waterstones book shop on Albion Street, signing copies of her new book Unbelievable for hundreds of fans.
While she was there she met Paul and Emma Kendrick, from Morley.
They have launched a campaign to make more people aware of Non-Ketotic Hyperglycinemia (NKH), a condition which disrupts the function of the brain.
The couple’s son Joseph suffers from the condition and they are hoping to help fund research that may one day find a cure.
Paul, 36, said: “Not many people have heard of NKH because it’s so rare and we want to let people know what it is and that it’s such a terrible condition.
“At the moment there’s only one doctor in Colorado that’s doing research and any money we can raise will go straight to this man who knows what to do with it.”
When Joseph was born, doctors feared he might struggle to live beyond six months.
But, three years later, Joseph’s Goal has received the backing of players from the Leeds Rhinos, Wigan Athletic Football Club and comedian and Mock The Week star Alun Cochrane.
For more details on the campaign, follow @josephsgoal on twitter, or visit www.josephsgoal.org.