A survey of children's favourite foods showed some contain double the recommended dose of salt or fat.
Fat in some foods will lead to 'obesity timebomb', say experts
The Trading Standards Institute warned parents to be on the look out for the huge variation in the amounts of fat and salt contained in some foods.
And it called on the food industry to standardise content labels in a readily understandable format.
Trading standards officers warned of an 'obesity timebomb' after analysing 279 foods popular with children.
The foods included breakfast cereals, lunch box foods, crisps, desserts, sweets, ready meals, pizzas and tinned products such as beans and spaghetti hoops.
They found a wide variation in nutritional values in similar foods - for example, some sweets contained 33g fat but others only had 0.1g, and the amount of salt in ready-made meals varied from 6.9g to just a trace.
In the course of one day a child eating the highest fat content breakfast cereal, snack, ready-made lunchbox, cereal bar, chocolate bar, ready-made meal, dessert and drink could be consuming 133.7g of fat.
The guideline daily amounts of fat are 85g for boys aged 11 to 14 and 70g for girls, including a maximum of 25g saturated fat.
If children ate food with the highest salt content they could be consuming 13.5 g of salt - more than twice the maximum recommended level for 11 to 14-year-olds and three- and-a-half times the amount for four to six-year-olds.
"Our survey indicates that there is a wide difference in the amounts of fat and salt present in similar types of foods," said Phil Thomas, TSI spokesman on food.
"Parents should check the amount of fat and salt when choosing pre-packed food for their children - and look at the actual amount that will be consumed by eating that product, rather than the content per 100g," he said.
He commended the Food Standards Agency for its 'traffic light' initiative, which encourages supermarkets and manufacturers to use a 'red, amber, green' nutritional guide on the front of packs to indicate levels of fat, saturated fat, salt and sugar
Professor Graham MacGregor, Professor of Cardiovascular Medicine and Chairman of Consensus Action on Salt and Health (CASH), said: "I find it shocking that food manufacturers are still producing foods aimed at children that contain large quantities of salt. "We know that salt acts as a chronic long-term toxin, slowly putting up blood pressure as we grow older.
"This rise in blood pressure is the major cause of strokes, heart failure and heart attacks."
Dr David Haslam, GP and clinical director of the National Obesity Forum said: "There are two problems, one is with education of the general public and one is food labelling."
"The traffic light scheme is done with good intentions but I don't think by itself it is the answer. We need education in schools and public information campaigns."