Parents need to be more aware of how often their children are having fast food.
Pediatrics Dietician in Temple Street Children's Hospital, Kizzy Moroney, said it has become a major issue.
Figures from the latest Growing Up In Ireland study show that one in four three-year-olds and primary school children are overweight or obese.
Ms Moroney says fast food and sweets are the main culprits, so it is critical parents are on top of their child's diet.
She said: "Having the control of how often it's happening you can really limit that, saying 'look you have four days until that party, let's not go to that place now, let's cook at home and have a healthier option. Help me cook'.
"Getting children involved and teaching them skills around cooking and exercise."
She also said it is important parents know of how regularly their children are having fast foods.
Ms Moroney said: "I think having the awareness of how often things are happening really will empower people. You have figures out where a range of kids were reviewed and their parents weren't aware that they were obese.
"34% of the parents of these obese children thought they were okay for their height, so it's very difficult.
"Awareness is the key, the information is out there, dieticians are available and your GP can help."