The brand, owned by Danone Baby Nutrition, handed the major brief to Frank after a competitive pitch.
Cow & Gate had previously used Brave PR for consumer PR, but launched a pitch process earlier this year as Brave's co-founders decided to voluntarily close the agency.
Frank's work kicks off with the UK's first 'Toddler Census', aiming to explore what life is like for British toddlers, from the contents of their wardrobes to their diets and lifestyles.
The campaign focuses on Growing Up Milk, a milk specially made for toddlers.
It is thought that the growing 'mummy blogger' community has been identified as a key target, given its growing importance as an information source for parents.
The campaign will blend traditional media relations with digital, SEO and social media activity, as well as a 'mum' word-of-mouth campaign.