Dundas was handed the brief after a non-competitive pitch as the charity more than triples its fundraising target for the event on 12 December from £1m last year to £3.5m.
The campaign will involve a co-ordinated PR effort to help get the public to wear festive knitwear and donate £2, with a particular focus on those aged 25 to 35.
Save The Children head of PR Helena Wiltshire said: "We have a PR plan produced by the in-house team and we needed someone to amplify it and help with a real media push on this.
"The campaign has been going for three years and wasn’t planned as a long-term fundraiser but became so successful we realised we could capitalise on it."
Social media will play a key element in the comms efforts, she added, with corporate partnership and work with celebrities also key.
Last year’s six-week campaign involved work with House PR and generated more than 1,400 pieces of coverage.