The agency, which specialises in work for not-for-profit organisations, will be responsible for improving registration levels for Barnardo's annual Lloyds TSB-sponsored fundraising event, which aims to break the £2m barrier next year.
Barnado's organises a number of regional Big Toddles taking place during the summer, as well as allowing people to organise their own events.
Barnardo's said Burnett Works would be working closely with the charity's in-house team to improve registration, conversion and retention rates across all audiences for the annual Big Toddle event, which last year involved 250,000 pre-school children walking half a mile to raise money for under privileged youngsters.
Gillian Egan, account director at Burnett Works, said: "Our aim is to grow income from next year's Barnardo's Big Toddle to £2.1m and to develop closer, long-term relationships between participants and Barnardo's."