The account, which has lain dormant for some years, will be handled by the agency's specialist children's division, MPG Kids, although Walker Media will continue to be a Mothercare roster agency.
MPG is working with St Luke's on the campaign, which breaks later this month. St Luke's won the creative account after Mothercare also saw pitches from Miles Calcraft Briginshaw Duffy and Rapier.
Earlier in September, OgilvyOne was handed a brief to overhaul Mothercare's customer communications and to develop a direct mail campaign and new data collection strategy.
A Mothercare spokeswoman said that MPG had only been appointed on a project basis because the company does not pay its agencies retainers.
MPG Kids was set up to handle all forms of specialist kids marketing, from strategy to implementation across traditional media buying and event and retail channels.
Separate to the Mothercare win, the division has also won the European media buying and online account for the US-based toy manufacturer Funrise. The company manufactures children's toys under brand names such as Tonka, Disney and Scooby Doo.
Kate Cox, MPG's joint head of strategy, said: "Kids marketing is an area often left under-resourced by many media and creative agencies, but it needs specialist advice and consultancy to really make a difference on smaller marketing budgets".
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