Double gold medal winner Rebecca Addlington OBE is expected to meet around 1,000 children from St Albans as part of the Legacy Games.
The event will be held at the Westminster Lodge Leisure Centre on Thursday, June 19.
Miss Addlington said: "Getting kids active is vitally important and the Legacy Games Day engages them in a fun way.
"I want to encourage the children to try something new and try their best. Without encouragement none of us gets too far so my aim will be to spur them on to do their best."
She will be taking part in the mass warm-up alongside all of the children, and will then spend time with the children in groups as they take part in a series of games and sports.
The Legacy Games Day , in association with Fit For Sport, is an organisation dedicated to bringing sport and fitness to children and young people across the country.
Lord Sebastian Coe said of the event: "I am delighted to support Fit For Sport and Everyone Active for their ‘Legacy Games’.
"I hope that this programme continues to grow and support communities, schools and families to become more active. It is a showcase of the lasting Olympic and Paralympic Legacy and one that I hope will encourage others to join in and move more."