Fat can be included as part of a healthy, balanced diet and has an important role in helping youngsters grow, they said.
The advice came after women were told not to eat junk food while pregnant as it can set their child's taste for fatty and sugary food for life, putting them at risk of obesity.
Other studies have said children should not be given low-calorie diet food because it could make them over-eat to compensate.
The new research, published in the Nutrition Journal, found that children's bodies burn a greater proportion of their energy from fat than adults do.
Experts said the way to make sense of all the latest evidence was to eat a sensible balanced diet without restricting any one food group and to take regular exercise.
An American team, led by John Kostyak from Pennsylvania State University, carried out tests on 10 children and 10 adults. They found that the total fat burned by children did not differ greatly to that burned by adults.
However, the children were found to burn considerably more fat relative to the amount of energy they used.
They had been given a standard diet for three days and confined to sitting around watching DVDs or reading, with measurements taken throughout. Women and girls used fat at a higher rate than men and boys.
The authors said, "Sufficient fat must be included in the diet for children to support normal growth."
The report said a quarter to a third of a child's calories should come from fat, compared with a fifth for adults.
Tam Fry, a board member of Britain's National Obesity Forum, said, "Young children need more fat and energy for the whole purpose of growing up and living. Therefore to give them low-fat and sugar-free products is a bad idea.
"Certainly right through primary school, children should be given straightforward wholesome food.
"It is difficult to address this issue because the whole business of food is so full of mixed messages.
"But the basic message is if the word 'diet' is written on a packet or can, it really shouldn't be given to children.
"Diets are not appropriate for young children. A certain amount of fat is a good thing, but don't overdo it.''
* The Foods Standards Agency says children under the age of two should be given full fat milk and dairy products. Semi-skimmed is fine once they are older. Fully skimmed milk should not be given to children under the age of five.
Parents should provide foods such as citrus fruit, green vegetables, milk, cheese, yoghurt, soya beans, nuts, fortified cereals, margarine or butter, meat and oily fish. Sweets, biscuits and salty snacks should be limited, according to FSA advice.
From the August 27, 2007, Prepared Foods e-Flash