The organisation said the roll-out of 1,140 hours of free childcare "has largely failed Scotland’s childminding workforce".
The body insisted a "step change in action" is now urgently required.
Funded early learning and childcare (ELC) is available to all three and four-year-olds and eligible two-year-olds, with the entitlement rising to 1,140 hours a year from August.
The SCMA was commissioned by the Scottish Government to undertake an annual audit charting the progress of all councils in including childminders in the roll-out.
Chief executive Graeme McAlister said: "The childminding workforce has declined by 26 per cent in the last five years – and this has accelerated in parallel to ELC expansion.
"This cannot be sustained and has significant implications for families, access to childcare and parental choice.
"It could also threaten the Scottish Government’s ability to deliver on it’s commitments in the Programme for Government to extend ELC downwards to one-year-olds and to develop a new system of wraparound school-aged childcare – both areas in which childminders are heavily involved and will play a vital role.”
Scottish Liberal Democrat education spokesman Willie Rennie said: “That such a low proportion of childminders are engaged in the newly expanded hours should be a wake-up call for the SNP Government.
“Childminders in Scotland have a wealth of experience and can provide a more homely, personalised, flexible offer, which is ideal for many children and their families.
“Nicola Sturgeon repeatedly promised flexibility for families accessing the free early learning and childcare hours, but the system is like a straitjacket.”
A Scottish Government spokeswoman said: “Childminders are a valued part of our ELC sector, providing high-quality childcare in a nurturing, home-based environment.
“The audit found that a majority of parents surveyed are receiving their first choice of childcare provider.
"However, we recognise that the report highlights some challenges for the childminding sector.
"We will consider the findings in detail and work with our local government partners to ensure councils are supported to further develop their ELC offers in response to parental demand and local circumstances.”