Some breakfast cereals contain as much sugar as chocolate bars, while others can match fat levels found in a bacon roll, research suggests.
Some puffed wheat cereals were low in sugar, fat and salt
More than three-quarters of cereals tested were high in sugar, the report by consumer group Which? added.
Asda's and Morrisons' Golden Puffs topped the sugar content league, with the report finding that both cereals had 55g of the ingredient per 100g.
Researchers said there was little improvement since their 2004 study.
Figures showed 88% of the 52 products specifically targeted at children were high in sugar.
Five of the worst offenders
Jordan's Crispy Nut Four Combo
Nestle Cinnamon Grahams
Quaker Oatso Simple Kids
Nestle Cocoa Shreddies
Kellogg's Fruit and Fibre
Experts found that 19% of all cereals tested had high levels of salt and 7% contained high levels of saturated fat.
Kellogg's Coco Pop Straws had similar sugar levels to a two-finger Kit Kat, while Sainsbury's Crunchy Oat Cereal had similar fat levels to its own brand of thick pork sausages, Which? said
Jordan's Crispy Nut Four Combo had the highest fat content at 28.5g per 100g while Kellogg's All-Bran and Morrisons' Right Balance had the highest amount of salt per suggested portion size.
The fat in the Jordan's product is the same amount per serving as a McDonald's McBacon roll, the report said.
Researchers said that the three worst children's cereals were Quaker Oatso Simple Kids, Kellogg's Coco Pops Straws and Mornflake Pecan and Maple Crisp.
Five of the healthiest cereals
Nestle Shredded Wheat
Quaker Puffed Wheat
Tesco Value Wheat Biscuits
Sainsbury's Basic Muesli
Asda Good for You Apple, Blackberry and Raspberry Flakes
They would all get "red lights" for sugar content under the Food Standards Agency's new labelling scheme.
Which? chief policy advisor Sue Davies said: "At a time when there is growing concern about childhood obesity and diet-related disease in general, it is simply not good enough that cereals marketed directly at children were among the worst offenders for sugar and for some in salt."
The consumer group's researchers surveyed 275 different types of cereals from leading brands and the UK's four largest supermarket chains.
They looked at the amounts of sugar, salt and fat per 100g and compared these to Food Standards Agency guidelines to work out high, medium and low content.
The Association of Cereal Food Manufacturers said breakfast cereals contribute a "nutritionally insignificant" amount of fat to the average diet.
A spokesman told the Daily Telegraph: "On average, cereals also contribute less than six per cent of the average daily sugar intake in children.
"There is no evidence to show that breakfast cereals make a significant contribution to energy, fat or sugar in the diet of the UK population."