Addressing the Scottish LibDem conference in Glasgow, Rennie accused the First Minister of denying nursery education to two-year-olds in order to maximise a Yes vote next year.
Rennie said that, while 130,000 two-year-olds in England now get 15 hours of free nursery care a week, Scottish children miss out.
He said the First Minister was using better childcare as "bait for voters", linking improvements to independence.
Salmond has previously promised a "transformational change" in childcare, arguing it would enable more mothers to return to work and boost the economy.
But because the extra tax revenue would currently go to the Treasury, not Holyrood, the First Minister has also said such a change should happen under independence.
Rennie said: "Alex Salmond will still not listen to our pleas to make it happen now. Instead he uses nursery education as a bait for voters.
"He tells us that Scotland will have the best childcare arrangements in these islands, but only after independence.
"They are demanding nothing short of a ransom from the voters before they release the benefit to our children.
"They are holding the futures of a generation of children hostage. It's not right."
The SNP condemned the inflammatory language as "hypocritical and tasteless".
It said the Scottish Government was spending £190 million to give 121,000 children more nursery hours, saving parents about £700 a year.
SNP MSP Clare Adamson said: "Not only is this attack highly hypocritical, it has also been done using particularly tasteless language.
"We have already increased free nursery provision by 20% since 2007 when we came to power.
"Expanding nursery provision from 475 hours to a minimum of 600 hours will be an increase of 45% since 2007, benefiting around 120,000 children in Scotland from 2014."
While three and four-year-olds are entitled to free nursery education, only a few hundred "looked after" two-year-olds (those in the care system) receive it in Scotland.
With the polls predicting a LibDem rout at the next election, Rennie also tried to raise activists' spirits by reminding them LibDems were often friendless.
"Scottish Liberal Democrats, and Liberals in Scotland, have often been a lone voice," he said."And this has never, and will never, intimidate us."