Cancer treatments can affect appetite
Three quarters of parents are critical of the meals served to their children in cancer wards, a survey suggests.
And 90 of the 92 parents and carers questioned said they had taken extra food into their child's hospital.
The charity CLIC Sargent, which carried out the poll, wants a "food champion" on every child cancer ward.
The Hospital Caterers Association said mealtimes were harder for distressed or sick children - but kitchens should be able to cater for their needs.
The powerful anti-cancer treatments given in hospital can have an effect on the appetite of patients, both child and adult.
Some 77% of those questioned by the charity described hospital food as "unappealing", "unsuitable for sick children" or "poorly prepared".
Nine out of 10 CLIC Sargent staff based in hospitals had received complaints from parents about the food on offer, and 41% rated the food available at their hospital as either "poor" or "very poor".
A group of children interviewed by the charity also criticised the menus at their hospitals, with some describing them as "toxic", "disgusting", or complaining that the food was still frozen.
All but 2% of the parents and carers interviewed said they had taken food into hospital to supplement the meals provided there.
CLIC Sargent chief executive Carole Easton said: "No parent should be compelled to bring in their own food.
"Urgent action is needed to ensure that the NHS is meeting the specific food needs of children and young people with cancer."
Dr Easton called on hospital trusts to appoint one existing member of staff on child cancer wards - perhaps the housekeeper - as a "champion" whose job would include getting to know the likes and dislikes of children, and encouraging them to eat the food they need.
Mike Stevens, a professor of paediatric oncology at the University of Bristol, said that a good diet during treatment was vital for children with cancer.
"It helps them to feel better during treatment and is a significant factor in keeping their immunity up against infections.
"I can see the difference in children on the wards who have been able to maintain their intake of good food."
The Department of Health and the National Institute of Health and Clinical Excellence have both introduced guidelines aimed at raising the quality of food offered to young cancer patients.
However, the charity said many hospital trusts were not following the guidelines.
Hospital Caterers Association chairman Neil Watson-Jones said that it was often difficult to cater for child patients, who might be upset during their hospital stay or put off their food by powerful treatments.
"The important thing, I have to say, is to get children to eat something, even if it's not the healthiest thing on the menu.
"The idea of a food champion is a sensible one - this is all about good communication between the ward and the catering department, and in some places, perhaps that isn't happening as well as it could be.
"Generally, if someone makes a request for something served in a slightly different way, the kitchens will be able to accommodate them, and there is normally an area in every ward where these little changes could be made."
A Department of Health Spokesperson said the latest independent Healthcare Commission in-patient survey showed the vast majority of patients were satisfied with the food they receive.
"Good food is important for all patients and the Department of Health has recognised this as a priority issue.
"We expect every Trust to take their responsibilities on hospital food seriously and to make sure that hospitals meet the standards that patients rightly expect."