Ministers will this week launch a campaign Be Safe, Be Sure, urging such informal foster parents to register officially.
Private fostering is when a parent arranges for a child under 16 to be looked after by an adult who is not a close relative or an approved foster carer for more than 28 days. This could apply to an adult looking after a young relative because their parents cannot do so, a neighbour or friend acting in place of a parent who is ill or in hospital, and even the care of foreign exchange students on extended visits.
Another common route to private fostering is someone who starts looking after the child of a partner they are not married to, when their partner leaves them, or dies. It is also common for teenagers to live with someone else because they are estranged from family due to rows.
A report by the then Care Commission in 2010 revealed that only 16 such arrangements were known to councils across Scotland. However, if private fostering is as common in Scotland as it is in England, the Commission claimed, there should be more than 600 such families north of the Border.
The Be Safe, Be Sure campaign aims to improve understanding of private fostering and see more councils informed of private fostering arrangements to ensure the safety of children not officially known to the system.
Parents or carers involved in private fostering have a legal responsibility to inform local authorities but it is thought most are unaware of this.
Once notified of a private fostering arrangement, local social workers will carry out assessments, including disclosure checks on adults within the household, health checks on the child and private foster carer, and checks to ensure the child's educational needs are being met.
Minister for Children and Young People Aileen Campbell said: "Often private fostering arrangements become necessary in very difficult circumstances - a couple may be splitting up, there may be illness or death or a young person may be very far from home.
"These are exactly the times that we need to make sure that advice and help is available.
"Many carers may not be aware that they are obliged to notify or have any idea who they need to inform. I hope that this campaign will help many more families be safe and sure."
Doctors, teachers, social workers, dentists, police officers and children's charities will receive leaflets telling them about the campaign. Trisha Hall, manager of the Scottish Association of Social Work, said: "If we are going to "get it right for every child", this means offering support and help, as well as assessment and vetting, to people who step in to care for children when their own parents are not able to do so."
A spokesman for Children's Minister Aileen Campbell, said: "We want Scotland to be the best place in the world to grow up. Cormac Russell makes a number of points about the way society regards children and we will listen to what he has to say with interest."