According to the Children's Food Campaign, many parents are unaware of the composition of a healthy diet and, like their offspring, need educating.
Campaign coordinator Richard Watts said that there is a "worryingly high level of ignorance" about what constitutes - and the importance of - a healthy diet.
"There is a lot more work that needs to be done on educating parents about what makes up a healthy, balanced diet and therefore what kind of things should go in lunchboxes," Mr Watts claimed.
The campaigner insisted that it is "absolutely vital" to educate parents as well as children, adding: "Educating the kids on their own is not enough."
Regulations introduced by the government last year state that school dinners must be free from poor-quality meat, fizzy drinks, crisps and confectionary.
In addition, every meal must include two portions of fruit and vegetables.