The scheme, which is due to come into force after the 2015 election, will help around 1.9 million families, with children aged under 12 where both parents work, at a cost of around £750 million.
Save the Children's Head of Scotland, Neil Mathers, said: "Poorest children were hit hardest in the recession and there is a huge risk of leaving them behind in the recovery. The childcare announcements are a step forward but they were isolated in a Budget that mainly focused on businesses and savers."
Matthew Reed, chief executive of The Children's Society, said it would make a "huge difference" to hundreds of thousands of the UK's poorest families.
Calling for more details, Mr Reed added: "Providing the poorest working families with at least 85% of their childcare costs will make sure childcare is affordable to those who need it most and making sure work pays."
However, Grahame Smith, Scottish Trades Union Congress (STUC) General Secretary, said the additional help with the childcare costs missed the point.
He said: "There is no control on childcare fees in the private sector and the new measures will mostly see public money going to meet fee increases.
"Low pay and poor working conditions for childcare workers remain unaddressed. Working parents need a serious investment programme for publicly funded childcare services, appropriate to the needs of local communities and workers in every industry - with no judgements about eligibility."
Prime Minister David Cameron said voters would look at Labour's lack of a similar plan.