Jackie Brock, the chief executive of Children in Scotland, said the country is "increasingly out of step" with other nations in Europe on the protection offered to children.
Anne Houston, of Children 1st, said change is needed to help make Scotland the "best place for children to grow up".
They were speaking after Staffan Janson, a Swedish expert on children's well-being, told how banning smacking had brought about a culture change in his country.
Sweden was the first country in the world to outlaw smacking in 1979. Mr Janson says parents now adopt different methods for disciplining their youngsters."
Mr Janson, a professor of social paediatrics at the University of Karlstadt and Orebo, told Holyrood's cross-party group on children and young people: "Scandinavian parents have learned in one generation to completely refrain from spanking their children."
Current law in Scotland bans parents from shaking their children, hitting them on the head or using implements to physically punish them. But they can argue that it was a "justifiable assault" if they punish a child by hitting them.
Ms Houston said: "It is incredible to think that we accept that the smallest and most vulnerable members of our society are the only ones whose assault is considered 'justifiable'.
"Today, we have heard how changing the law, along with providing support for parents to find other ways to discipline their children, not only protects children from assault but positively changes family relationships too."
A Scottish Government spokeswoman said it "does not support smacking". "However, we do not wish to criminalise parents for lightly smacking their child. The Criminal Justice (Scotland) Act 2003 clearly outlines what is unacceptable punishment," she added.