Geronimo was appointed following a three-way pitch and has been asked to create loyalty campaigns for BBC Magazines' key children's titles, including CBeebies Weekly, Dr Who Adventures and Girl Talk.
It is understood the work will take in direct marketing, online and press advertising.
Jess Burney, subscriptions director at , said: "We were very excited by the creativity that showed in the pitch. The standard was exceptionally high but Geronimo had the edge in original thinking."