Powell also called for the two-child benefit cap to be lifted "in full", saying this was the only way to ensure child poverty falls significantly over the course of the Parliament.
She added that the issue was "urgent" because every year the policy is in place at least 40,000 children "are pushed into deep levels of poverty as a result".
The government has been facing growing pressure from its own MPs to lift the cap, which restricts child tax credit and universal credit to the first two children in most households.
Reeves is expected to make changes to the cap, which was introduced by the Conservatives, in her Budget.
However, experts estimate that scrapping it completely would cost around £3.5bn a year.
Other options reportedly being considered by the Treasury include raising the cap to cover three or four children, exempting families where at least one adult is in work or reducing benefits for third and subsequent children.
Campaigners have called for it to be abolished in full, arguing this would be the most cost-effective way to reduce child poverty.
