The number of under-18s "at risk of poverty or marginalization" — an official EU measure of various aspects of economic hardship — soared to more than 2.8 million in 2012, the charity said.
That was equivalent to 33.8 percent of Spain's children, it said in a report that used the latest official European Union data.
Spain began in the third quarter of 2013 to crawl out of its second recession in five years. The double downturn has thrown millions out of work after the collapse of a building boom.
The jobless rate remains extremely high at more than 26 percent, with nearly six million Spaniards unemployed.
Save the Children cited cases of families scraping by on low salaries, dwindling welfare payments and charity food handouts.
The conservative government that took power in late 2011 imposed tough public spending cuts in its efforts to stabilise Spain's public finances, under pressure from European authorities.
"Austerity policies are considerably worsening the situation," Save the Children said in its report.
They are "constraining still further the already limited capacity of the social welfare system to give an adequate response to the needs of children and families in economic difficulty", it added.
"The state has an obligation to act as guarantor of the rights these girls and boys are entitled to, an obligation at an international level, defined in human rights treaties."