A child throws up twice in the morning, refuses lunch, and goes quiet on the sofa. Most of the time this is a 24-hour stomach bug that resolves with patience and a bit of fluid. But children — especially under 5 — can move from "off colour" to clinically dehydrated faster than adults expect, and the signs that mark the difference are specific enough that they're worth memorising before the moment you need them. The two things that change outcomes most: knowing which signs are real red flags, and using the right fluid. For more on common childhood illnesses, visit Healthbooq.
Why Children Dehydrate Faster Than Adults
A child has more body surface area per kilogram than an adult, which means they lose more fluid through skin and breathing — particularly when they have a fever. Their kidneys are less efficient at concentrating urine when fluid is short. Infants under 1 have especially high baseline fluid requirements (around 100 ml/kg/day). And young children can't reliably tell you they're thirsty.
Gastroenteritis is by far the most common cause of significant dehydration. Rotavirus, before the UK introduced the vaccine in 2013, was the leading cause of severe infant gastroenteritis; norovirus and bacterial infections still produce plenty of cases. Vomiting and diarrhoea together is the dangerous combination — losses come from both ends, intake stops, and a small child runs out of buffer fast.
Fever alone, even without gut symptoms, increases fluid losses substantially. A child with a 39°C fever needs about 12 per cent more fluid than usual just to stay even.
The NICE Framework: How Doctors Assess Dehydration
NICE CG84 (Diarrhoea and vomiting in children under 5) breaks dehydration into three clinical categories that are useful at home too.
No clinically detectable dehydration. The child is alert, behaving normally, eyes look normal, mouth and tongue are moist, tears are present when crying, urine output is normal, and skin springs back immediately when gently pinched.
Clinical dehydration (5–10 per cent fluid deficit). Behaviour is altered — irritable, less responsive, not themselves. Eyes look sunken. Mouth is dry or sticky. Tears reduced. Urine output reduced. Skin recoils a bit slowly when pinched. Heart rate is faster than usual. The child is unwell but not in shock.
Shock (over 10 per cent fluid deficit). This is an emergency. Decreased consciousness, very sunken eyes, no tears, cold and pale or mottled extremities, fast and weak pulse, capillary refill greater than 2 seconds (press a fingernail or the chest until it blanches; colour should return within 2 seconds), very rapid breathing, and very reduced or no urine output. Call 999 or go straight to A&E.
The skin pinch and capillary refill tests are useful at home and take seconds. The capillary refill test is particularly worth knowing — press on the centre of the chest or a fingernail for 5 seconds, release, and count how long until colour returns. Under 2 seconds is normal; longer is concerning.
Urine Output: The Most Reliable Home Indicator
If you remember one home metric, make it urine output.
- A baby under 1: at least 6 wet nappies in 24 hours is reassuring; fewer than that suggests dehydration is developing.
- An older child: at least one urine output in 6–8 hours, with normal colour (pale yellow). Dark yellow or amber urine, especially when concentrated and small in volume, is a warning.
- No urine for 8 hours in any child warrants urgent assessment.
Tears when crying is the second easiest check. A child who would normally tear up but isn't is dehydrated until proven otherwise.
What to Give: ORS, Specifically
Oral rehydration solution (ORS) is what works. The reason: it contains glucose and sodium in proportions that activate the gut's glucose-coupled sodium transporter, which pulls water across the gut wall along with the salt. Plain water doesn't have this effect and dilutes blood sodium. Fruit juice has too much fructose and worsens diarrhoea. Sports drinks have the wrong glucose-to-sodium ratio. Flat cola is a folk remedy with no clinical basis.
Available ORS products in UK pharmacies without prescription: Dioralyte, Electrolade. In the US: Pedialyte. Always mix with the exact volume of water on the packet — using less water makes the solution dangerously concentrated.
NICE CG84 recommends roughly 50 ml/kg of ORS over 4 hours for clinical dehydration, on top of maintenance fluids and ongoing losses. For a 15 kg three-year-old, that's about 750 ml over 4 hours.
For a child who is actively vomiting, the trick is small volumes very frequently — 5 ml every 2–3 minutes from a syringe or medicine spoon. A vomiting child can almost always keep down 5 ml even when a full cup comes straight back up.
Food and Diet During Illness
NICE recommends resuming normal diet as soon as the child is willing. The old "BRAT diet" (bananas, rice, apple, toast) is no longer recommended as specifically beneficial — there's no evidence it helps over normal eating. Fluid is the priority; food can wait until they're hungry.
Breastfed babies should keep breastfeeding throughout. Formula-fed babies should keep their normal-strength formula — diluting it is an outdated practice that makes things worse.
What Not to Do
No anti-diarrhoeal medicines (loperamide, others) in children under 12 — NICE CG84 explicitly recommends against. The diarrhoea is clearing the infection.
No anti-emetics at home without medical advice. Ondansetron is sometimes used in A&E for persistent vomiting, but home use isn't appropriate.
No probiotics for routine treatment — the evidence is inconsistent and they're not part of NHS guidance.
When to Get Help
Call 999 or go to A&E for any signs of shock: decreased consciousness, mottled or cold extremities, capillary refill > 2 seconds, very weak or rapid pulse, no urine for many hours, a child who is floppy or extremely difficult to rouse.
Same-day urgent assessment (NHS 111, GP, or A&E) for:
- Any baby under 3 months with fever and reduced feeding — get them seen the same day regardless of how mild it looks
- Any baby under 6 months becoming dehydrated
- Cannot keep any fluid down for 8 hours
- Signs of clinical dehydration that aren't improving with home ORS
- Bloody or persistently green diarrhoea
- Severe abdominal pain
- A child with a known condition — type 1 diabetes, congenital heart disease, kidney disease — who has any vomiting and diarrhoea
- High fever in any child plus poor fluid intake
If you're unsure and the child looks unwell, call 111 or your GP. "I'm worried" is a perfectly legitimate reason to seek advice.
Heat and Exercise Dehydration
In summer or during sport, children lose substantial fluid through sweating and don't always recognise their own thirst. Practical measures: scheduled water breaks every 15–20 minutes during outdoor play in hot weather, light loose clothing, shade, and avoiding the hottest part of the day. For routine heat exposure, plain water is fine — ORS is for illness with fluid losses, not for routine hydration.
Children with diabetes, kidney disease, or other conditions affecting fluid balance need closer monitoring in heat — talk to their specialist about specific guidance.
Prevention of Gastroenteritis
Hand washing with soap and water — not alcohol gel, which doesn't reliably kill norovirus. Rotavirus vaccine on the NHS at 8 and 12 weeks. Strict 48-hour exclusion from nursery, school, and group care after the last episode of vomiting or diarrhoea (NHS and UKHSA guidance). Once a stomach bug is in the household, treat door handles, taps, and toilet flush handles as contaminated and disinfect daily.
Key Takeaways
Most childhood dehydration is mild and corrects at home with oral rehydration solution. The signs that matter are specific and worth knowing in advance: no urine for 8 hours, no tears when crying, sunken eyes or fontanelle, capillary refill greater than 2 seconds, and a lethargic or hard-to-rouse child. The right fluid is ORS — not water, juice, or sports drinks. Severe dehydration is a 999/A&E emergency.
