Parents are not legally obliged to use the Scottish Government's named person scheme but it is right that the service is universal, the First Minister has said.
Nicola Sturgeon said the measure, which is currently being challenged in the courts, was aimed at "stopping children falling through the net" and did not amount to a "state guardian" being appointed for every child.
Conservative leader Ruth Davidson said Ms Sturgeon was trying to "spin her way out" of an "unpopular" policy.
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Under the scheme, a single point of contact, such as a teacher or health visitor, would be assigned to look out for the welfare of children under 18.
The named person is required to exercise statutory functions, including providing advice, information or support where appropriate to promote, support or safeguard the wellbeing of the child or young person.
A poll conducted earlier this month found almost half of people in Scotland opposed the plan while less than a third supported it.
During First Minister's Questions at Holyrood, Ms Davidson asked: "Are parents who don't agree with this scheme able to stop their child from having a named person and withdraw their child from all named person provisions?"
Ms Sturgeon responded: "The named person scheme is an entitlement, I think it is a good and sensible entitlement. It is not an obligation.
"It helps children and families get the support they need from services when they need it.
"It does not in any way, shape or form replace or change the role of the parent or carer or undermine families."
Referencing the universality of the scheme, she added: "It is not possible to predict in advance which children might become vulnerable."
Ms Davidson said: "Named person legislation is so sweeping and now so unpopular that it is no wonder that the First Minister is trying to spin her way out of it.
"But isn't it dishonest to suggest that a parent choosing not to engage with a named person is the same thing as being able to stop their child having one imposed in the first place?"
Ms Sturgeon insisted: "The fact is children and parents are not legally obliged to use the named person service or take up any of the advice or help that is offered to them, but it will be available to them if they need it at any point in the future."