As part of a one-year Spending Round announced by Chancellor Sajid Javid on Wednesday, the extra money will be provided to boost the hourly rate paid to providers of the 30 hours childcare offer.
But the Early Years Alliance, which represents 14,000 early years settings, said the money will plug just 10 per cent of the estimated £662m funding shortfall in the sector.
It is calling on early years professionals to write to their MPs urging them to highlight local circumstances to Whitehall civil servants.
Neil Leitch, chief executive of the Early Years Alliance, said: "[The] funding announcement was not the serious response to the early years funding crisis the sector had a right to expect.
"In fact, it will be less than nothing to those providers who have endured real-terms cuts to funding levels in recent years and will anyway be completely meaningless once next April's minimum wage increase kicks in.
"I know many providers will be sick to the back teeth of campaigning by now - but we need the government to know that this level of increase is tokenistic and unacceptable.
"The best way to do that is by asking our local MPs to speak to the Treasury and the DfE about what this means in their constituencies - that is why I'm urging every provider to pick up their pens and ask their MP to speak up for them."
The government's 30-hour childcare offer for working parents of three- and four-year-olds was introduced nationally in September 2017, but the early years sector has consistently warned that the amount of funding from government is not enough to cover the cost of providing it.
A DfE evaluation of the early implementation of 30 hours childcare in eight local authority areas, published in July 2017, highlighted the financial impact of delivering the extended offer.
Liz Bayram, chief executive of the Professional Association for Childcare and Early Years (Pacey), described the £66m funding boost announced by Javid as "a start".
"However, the sector needs more, it is crying out for a long-term investment strategy that ensures providers can deliver the high-quality places children and families deserve without risking their childcare businesses' future sustainability," she added.
"This £66m, alongside the additional £700m for high needs children and young people in the school settlement will help, however, Pacey's focus remains on working with DfE and Treasury to ensure this acknowledgement that funding is too low is developed into sustained and full investment in the government's early education strategy.
"Only then will we start to see the disadvantage too many children still experience begin to reduce."
A delayed full multi-year Spending Review is due to take place in 2020.