Junk food makes up 20% of children's diets in the Irish Republic, leaving children short of essential nutrients, according to the Irish Nutrition & Dietetic Institute.
Parents and government should play more active roles in tackling the problem, it said, by ensuring youngsters have a balanced diet and banning TV ads for junk foods before 9pm.
INDI dietician Margot Brennan said: "Children have high requirements for growth and need calcium, iron and protein. If 20% of their calories are coming from foods that offer fat and sugar, but not these nutrients, they're missing out.
"The problem lies not in any one junk food but in the cumulative effect. If children have a bag of crisps, then they'll have pop as well."
But the Snack Food Council of Ireland said that fat in products had been "significantly reduced".