A stomach bug runs through a nursery, and within 48 hours half the babies in the room are home with their parents. What surprises most first-time parents is how fast a small child can move from "off her food" to genuinely dehydrated — sometimes within a single afternoon of vomiting. The good news is that most cases stay mild and are completely manageable at home with the right fluid given the right way. The trick is knowing what to give, how much, and the specific signs that mean it's time to stop trying at home and get medical help. For more on common childhood illnesses, visit Healthbooq.
Why Babies and Toddlers Dehydrate So Fast
A newborn's body is roughly 75 per cent water; an adult's is closer to 60. That difference matters in both directions — babies have proportionally less reserve and lose fluid faster through their skin and breathing because of their large surface area relative to body weight. A 5 per cent fluid loss is enough to be clinically dehydrated, and that can happen in a few hours of vomiting and diarrhoea in a small child.
Breastfed babies have a real edge during illness — breast milk gives them fluid, electrolytes, and antibodies, and most will feed more often when unwell. Formula-fed babies and toddlers eating a normal mixed diet don't have that buffer.
What Dehydration Looks Like
Mild dehydration is subtle. The baby's urine output drops a bit and their mouth feels less wet than usual, but they still play, smile, and feed reasonably. This is the easiest stage to correct and the one most parents handle without realising they did.
Moderate dehydration is visible. Specific things to look for:
- Fewer than 6 wet nappies in 24 hours in a young baby (or noticeably fewer than their usual count)
- Urine that is darker yellow than usual, or a strong smell
- Dry, sticky-feeling lips and tongue
- No tears when crying — a baby who would normally tear up has eyes that stay dry
- Eyes that look slightly sunken
- A sunken fontanelle (the soft spot on the top of the head in babies under 18 months)
- Less active, more irritable, harder to settle than usual
Severe dehydration is an emergency. Signs include no urine for 8 hours or more, eyes and fontanelle deeply sunken, very dry mouth, skin that stays "tented" briefly when you gently pinch it, mottled or cold hands and feet, capillary refill greater than 2 seconds (press a fingernail or the chest until it blanches; colour should return within 2 seconds), fast or laboured breathing, marked lethargy, or a baby who is floppy and difficult to rouse.
Call 999 or go to A&E immediately for any of those. This is not a watch-and-wait situation.
The Right Fluid: ORS, Not Water
Oral rehydration solution (ORS) is what you want. Dioralyte is the most widely available brand in UK pharmacies; Electrolade is another. In the US, Pedialyte is the equivalent. The reason ORS works and plain water doesn't is that it contains glucose and sodium in proportions that activate the gut's glucose-coupled sodium absorption — basically, the sugar drags the salt and water across the gut wall together, even when the gut is inflamed.
A few important rules:
- Mix ORS with the exact volume of water on the packet. Too much water makes it ineffective; too little makes it too salty and unsafe.
- Use a fresh sachet — discard prepared solution after 24 hours (1 hour at room temperature in hot weather).
- Don't add sugar, juice, or squash to make it more palatable. The composition matters.
Plain water alone dilutes blood sodium without replacing what's lost. Fruit juice is high in fructose, which makes diarrhoea worse. Sports drinks have the wrong ratios. Flat cola is folklore — it has too much sugar and not enough sodium.
How to Give ORS to a Vomiting Child
Volume per swallow is the whole game. A vomiting baby cannot keep down a full bottle, but they can almost always keep down 5 ml. The technique that works:
- 5 ml every 2–3 minutes for a baby or toddler who is actively vomiting
- A 5 ml medicine syringe or spoon, into the side of the mouth
- Patient, slow, repeated — set a timer if you need to
Once they've kept it down for 30–60 minutes, gradually increase the volume and stretch the interval. NICE CG84 suggests roughly 50 ml/kg of ORS over 4 hours for clinical dehydration, given alongside maintenance fluids. For a 10 kg toddler that's about 500 ml across 4 hours — roughly 125 ml an hour, or 5 ml every 2–3 minutes if they're still being sick.
If a child is breastfed, keep breastfeeding — don't replace it with ORS, give ORS in addition. If they're formula-fed, full-strength formula is fine; do not dilute it (an outdated practice that makes things worse).
What About Food?
Old advice said "rest the gut" and avoid food. That's no longer recommended. NICE CG84 specifically says children should resume their normal diet as soon as they're willing — there's no benefit to the BRAT diet (bananas, rice, apple, toast) over normal eating. If a toddler wants toast and yoghurt, give it. The gut heals faster with food in it than without.
Avoid very fatty, very sugary, or very spicy foods for a day or two while things settle, but otherwise normal eating is fine. Don't force food on a child who isn't hungry — fluid is the priority.
When to Get Help
Same-day GP appointment, NHS 111, or pharmacy advice if:
- Any baby under 6 months with significant vomiting and diarrhoea
- Fewer than 6 wet nappies in 24 hours
- Vomiting persisting more than 24 hours, or diarrhoea more than 5–7 days
- Not improving despite ORS given correctly
- Blood or mucus in the stool
- Persistent severe abdominal pain
- High fever (over 38°C in a baby under 3 months, over 39°C in older babies)
- You're unsure and worried — that's a legitimate reason on its own
Call 999 or go straight to A&E for severe dehydration signs (sunken fontanelle, lethargy, mottled skin, capillary refill > 2 seconds, no urine for 8+ hours, very difficult to rouse), green vomit (suggests obstruction), or any baby who looks genuinely unwell beyond the immediate vomiting.
Avoid Anti-Diarrhoeal Medicines
Loperamide and similar anti-diarrhoeal medicines are not recommended for children under 12 (NICE CG84). Diarrhoea is how the body clears the infection — slowing it down keeps the pathogen in longer and increases the risk of complications without addressing the cause.
Prevention
Hand washing is the single most effective measure. Soap and water beat alcohol gel for norovirus specifically, because alcohol gel doesn't reliably kill it. The rotavirus vaccine, given on the NHS at 8 and 12 weeks as drops by mouth, has cut severe rotavirus gastroenteritis dramatically — UK rotavirus admissions in under-5s fell by around 70 per cent after introduction in 2013.
Once a stomach bug starts in a household, expect everyone to get it within a week. Keep separate towels, wash hands obsessively, bleach toilet handles and door handles daily, and know that the 48-hour exclusion rule (NHS guidance) for nursery is real — your child needs to be 48 hours clear of the last episode before going back.
Key Takeaways
Babies and toddlers can become significantly dehydrated within hours, especially with vomiting and diarrhoea. The right treatment for mild to moderate dehydration is oral rehydration solution (ORS) — not water, juice, or sports drinks — given in tiny, frequent amounts. Watch for fewer than 6 wet nappies in 24 hours, no tears when crying, sunken fontanelle, and a child who is hard to rouse. These last signs are an emergency.
