Free early learning and childcare is available to two-year olds from disadvantaged backgrounds, but the party said just 4,866 of an estimated 14,500 eligible children are taking advantage of the provision which the SNP claimed would help reduce the attainment gap.
It also follows warnings from Save the Children pre-pandemic of low uptake across Scotland.
Scottish Liberal Democrats leader Willie Rennie said the SNP were “setting up to fail another generation” after 14 years in power.
He said his party would extend the 1,140 hours of free childcare promised by Nicola Sturgeon’s party to all two-year olds.
Mr Rennie said: “Based on the SNP Government's best guess, there are 10,000 two-year-olds from deprived backgrounds losing out on free childcare. That's an almighty gap between what the SNP promised and what they are delivering.
"After 14 years in government, the SNP are setting up to fail yet another generation.
"They will no doubt have their excuses at the ready, but it's all bluster. Similar offers elsewhere in the UK have uptake twice as high.
"That shows how valuable this offer could be. The difference here is that we have a government which is distracted.
"The pandemic has weighed heavily on children and parents and we know that the early years are critical to life chances and future attainment. Getting the next five years right for our children and young people will be so important.”
Responding, an SNP spokesperson said in a statement: "The SNP is focused on steering Scotland through the pandemic and into recovery and a crucial part of that will be ensuring every child has the best start in life.
"Provision of free childcare currently rests with local authorities, with the majority of those in Scotland providing the full 1,140 hours for free. The full expansion is set to be completed by August this year.
"If re-elected the SNP will create a system of wraparound childcare, providing year-round care before and after school with low-income families paying nothing towards it."