The account, which has lain dormant for some years, will be handled by the agency's specialist children's division, MPG Kids. However, Walker Media will continue to be a Mothercare roster agency.
MPG is working with St Luke's on the advertising campaign, which will break later this month. St Luke's won the creative account after Mothercare also saw pitches from Miles Calcraft Briginshaw Duffy and Rapier.
Earlier this month, OgilvyOne won a brief to overhaul Mothercare's customer communications and to develop a direct mail campaign and new data collection strategy.
MPG Kids was set up to handle all forms of specialist children's marketing, from strategy to implementation across traditional media buying and event and retail channels.
Separately, the division has won the European media buying and online account for the US-based toy manufacturer Funrise. The company makes children's toys under brand names such as Tonka, Disney and Scooby Doo.
Kate Cox, MPG's joint head of strategy, said: "Children's marketing is often left under-resourced by media and creative agencies, but it needs specialist advice and consultancy to make a difference on smaller marketing budgets."