A NATS pledge to scrap the two-child cap will lead to an overall benefits bill of £9billion by 2030, a watchdog warns.
And ministers have been accused of risking “welfare tourism”, with families choosing to move to Scotland for up to a near £300-a-month boost.
The independent Scottish Fiscal Commission said the policy will also result in less cash for frontline services.
Its report says axing the two-child cap alone will cost almost £200million a year by the end of the decade.
The SFC says: “Where a family might otherwise have moved to England, or could make a move to Scotland, the mitigation payment would change the incentives.”
Scottish Tory social security spokeswoman Liz Smith said: “Under the SNP, the benefits bill is rising rapidly to unsustainable levels.
“There is potential for some to seek to move here to take advantage of SNP policies, raising the risk of benefits tourism, which would cost Scots taxpayers even more.”
The SFC estimates around 50,000 children will be eligible for the extra cash.
Under the SNP, the benefits bill is rising rapidly to unsustainable levels
Liz Smith
It warned a family with three children on the edge of being eligible could work less to stay on Universal Credit and keep an £8,000-a- year payment.
This could cost an extra £10million per year by 2030.
The watchdog said: “It contributes to the trend of social security spending taking up an increasing share of the resource budget.”
The payment would be at least £3,500 annually per additional child for families with more than two children if it is in place by 2026.
But this would rise to £3,845 by 2029/30 if benefits rise with inflation.
Scottish LibDem economy spokesman Willie Rennie said: “Our public services are taking a hit because the SNP government cannot get the economy growing.
“There needs to be a safety net, but the government must also ensure that as many people as possible can participate in the economy.”
Social Justice Secretary Shirley-Anne Somerville said: “We will continue to ensure our investment in social security is sustainable.”