Kate Bell, from the Child Poverty Action Group, said rising rent costs in London would mean the benefit cap hits families in the capital extra hard.
"We don't think it's fair to make children suffer for this cap.
"The real reason why people are going to be hit by the cap is because of the cost of their housing - most of the money these people are receiving in state support is going to housing benefit, which, of course, goes to the landlord, not to the family."
More top news
Police have arrested four men in connection to the suspected terror-linked murder of a police worker in Sydney.
Volkswagen could start recalling cars containing emissions cheating software in January, according to the firm's new chief executive.
A toddler survived for days on his own after his mother collapsed and died as she fed his one-year-old brother - who also choked to death.