Half of under fives have bouts of diarrhoea and vomiting each year
Parents are making children suffering from vomiting and diarrhoea more sick by giving them flat coke and lemonade, experts say.
The National Institute for Health and Clinical Excellence said it was a myth that sugary drinks could help ease bouts of gastroenteritis.
Instead, NICE said bad cases of stomach bugs in children under five needed to be treated with rehydration drinks.
The NHS advisers said prompt action was needed to avoid hospital admission.
NICE made the warning as part of guidance it has produced on the treatment of gastroenteritis in children in England and Wales.
Half of all children under five will develop vomiting and diarrhoea over the course of the year.
Up to a fifth will end up seeing a health professional about the illness with nearly 40,0000 children a year ending up in hospital because of problems related to dehydration.
NICE believes some of the most serious cases could be avoided if parents and GPs followed the best advice.
Consultant paediatric gastronenterologist Dr Stephen Murphy, who chaired the panel drawing up the guidance, said: "The idea that flat coke and lemonade - or fruit juices for that matter - helps is just a myth. In fact, it can make it worse, but unfortunately people are still using them.
"Severe cases of diarrhoea and vomiting leading to dehydration need treating with oral rehydration solution immediately."
He said the combination of sugar and salt in rehydration drinks was the key to helping the body absorb fluids, whereas the likes of coke and lemonade had too much sugar.
NICE has produced a checklist for parents to assess whether their children are dehydrated.
The key signs are altered responsiveness, sunken eyes, pale or mottled skin and cold extremities.
If they are, set amounts of oral rehydration solution should be given over the course of four hours.
The amount of solution to be given varies depending on the child, but for the average one-year-old it would be half-a-litre, the guidance said.
After that, it is important that children are encouraged to eat food again, NICE said.
The guidance is also aimed at doctors and gives advice on when to carry out further tests and when and how to administer intravenous rehydration fluid.
Mother-of-three Narynder Johal, who acted as a patient representative for NICE, said the guidance was much needed as parents were often left frustrated by the advice given to them.
"I have often been very concerned when my children have had diarrhoea and vomiting and have not always received consistent advice on how to best manage the condition."