The average British child drinks just half of the recommended amount of water, researchers said.
Children drink on average three 250ml glasses of water each day with less than a fifth (18%) actually consuming the recommended six glasses, they found.
And one in seven parents said their child does not usually drink any water at all, prompting experts to urge families to make sure youngsters stay hydrated in the heatwave.
More than one in ten (11%) parents gave their children fizzy drinks whenever they asked, while almost as many (9%) let their offspring have other sugary beverages whenever they were thirsty.
However, most parents recognised thirst and lethargy as symptoms of dehydration, and six in ten knew that a lack of water could trigger a fall in concentration or energy, according to the survey of more than 1,000 adults by drink brand Nestle Pure Life.
Nutritionist Anita Bean said: "As the hot summer months continue and with more hot weather predicted, it is even more important to ensure your children are well hydrated when they're running around keeping active.
"Children can easily forget to drink, particularly when they're active and having fun. In order to prevent involuntary dehydration, encourage them to drink little and often throughout the day.
"Water is essential for both adults and children to get rid of the body's waste products and to help their kidneys, brain and digestive system function properly."
Meanwhile, medical professionals called for an system to alert frail older people when a heatwave hits.
Doctors writing on the British Medical Journal website bmj.com recommended that vulnerable older people should drink two litres of water, milk, or fruit juice a day.
If they were taking drugs that suppressed their thirst, or disturbed the balance of fluids in the body, these should be temporarily stopped or the dose reduced.
Other measures suggested included educating those at risk to dress lightly, take frequent showers, and avoid alcohol.
The London Ambulance Service said its control room fielded 4,754 calls on Wednesday, signalling a 23% increase on the previous week.
The number of patients reporting difficulties in breathing rose by 34% compared to the same day last week, while calls relating to people who fainted soared 30%.
Deputy director of operations Jason Killens said: "The last few days have been very busy for both our control room and frontline staff and we would urge everyone to take care when they are out and about in the sun.
"Anyone with minor conditions should consider other healthcare options, such as calling NHS Direct on 0845 46 47 or visiting a minor injuries unit, NHS walk-in centre or local pharmacist.
"If they need to go to hospital for non-emergency treatment, they should use a taxi or get a lift from a friend or relative."