"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.
"This survey revealed that on average the foods had less than 20 per cent fat. However, there were examples of meals, snacks, sweets and other foods which contained more than 20 per cent, one of which contained 33 per cent fat," he said.
And the TSI is calling on the food industry to standardise labels that can be easily understood by the consumer.
The TSI commended the FSA's "traffic light initiative" to label levels of fat, saturated fat, salt and sugar according to the 'red, amber and green' system - a system generally opposed by manufacturers, who are reported to support the use of "Guideline Daily Amounts" (GDAs).
"Again there is an issue with consumers being able to make informed choices about the food they eat. The regulations do not require the amount of trans fats to be declared and saturated fat need only be declared when a claim about fat content has been made," said Thomas.
Trading standards officers from 37 local authorities in England and Wales purchased 279 samples of food aimed at children and determined the fat and salt content either by direct analysis of the product or by using data from the manufacturer.
Assuming a child eats all the highest fat products the TSI calculated that a child could have a fat intake of over 130 grams per day, almost double the 70 grams recommended for girls.
Dr Frankie Phillips, a dietician from the British Dietetic Association (BDA), the professional association for dieticians in the UK, said that it could be tricky to compare foods unless consumers looked carefully at the label.
"The amount of saturates is of particular concern because of the potential link with raised cholesterol levels and heart disease - and the amount consumed by children at the moment is simply too high," she said.
And the high salt foods could lead to a daily salt intake of 13.5 grams, more than three and a half times that recommended to four to six year olds.
But by eating the low salt content foods the daily intake could be slashed to just more than half a gram.
Professor Graham MacGregor, chairman of CASH and professor of cardiovascular medicine, said that the high salt products checked in the TSI survey were "literally poisoning our children's futures."
Professor MacGregor said that a high salt diet in childhood could lead to a range of health problems in later life, such as high blood pressure, osteoporosis, kidney stones, respiratory illness and stomach cancer.
"If some companies are able to make foods for children with only a trace of salt, why can't they all?" he asked.
The foods investigated included breakfast cereals, lunch box foods, snacks, crisps, desserts, sweets, chocolate, ready meals, biscuits, bars, pizzas and tinned products like beans and spaghetti hoops.
Ron Gainsford, chief executive of TSI, said that the diet of many children was a cause for concern.
"There is an increase in obesity and diabetes in young people and it is estimated that, if current trends continue, a third of adults, one fifth of boys and a third of girls will be obese by the year 2010.
We carried out our latest survey to highlight to parents that they can make choices which will give their children the chance of longer, more healthy lives," said Gainsford.