Rapier will develop creative and strategic activity to accelerate the growth of Barnado's biggest annual fund-raising event.
The Big Toddle is the largest event for children under five in the UK.
Last year, more than 120,000 children toddled half-a-mile to raise more than £1 million for the charity's Early Years Programme and local communities.
Barnardo's marketing director, Andrew Nebel, said: "Rapier demonstrated the ability to help create a step change in the success of the Big Toddle. It impressed us with its grasp of the issues and the opportunities, as well as its commitment to our aspirations for this event."
Barnardo's is currently without a direct marketing agency for its overarching fundraising activity after Partners Andrews Aldridge resigned the account in April.
The charity's advertising is with Bartle Bogle Hegarty. The agency created the controversial "silver spoon" campaign, which featured three images of babies with a syringe, a cockroach and a meths bottle in their mouths.