What signs of dehydration must not be ignored?

What the evidence says

Mild dehydration (occasional less-wet diaper, slight thirst) is common with a cold or upset stomach and usually improves with extra fluids. But some signs mean the body's fluid and salt losses have gone further and need a doctor's assessment, possibly the same day. Watch closely for: fewer than 4 wet diapers in 24 hours for infants (fewer than 3 for older children), no tears when crying, a dry or sticky mouth and tongue, a sunken soft spot on a baby's head, unusual sleepiness or difficulty waking the child, and breathing that seems fast or a heart rate that seems fast.

Vomiting green (bile-coloured) fluid is also an immediate emergency-department sign, not something to wait out at home. In babies under 6 months, refusing fluids for several hours, or a child of any age who cannot keep any fluids down and shows any of the signs above, warrants urgent medical review rather than home management. During hot weather, extreme thirst and unusually dark, scant urine are additional early warning signs Health Canada highlights for children in the heat.

A simple point-based tool called the Clinical Dehydration Scale, used by Canadian emergency departments, scores things like how alert the child looks, dryness of the mouth and eyes, and tears; higher scores mean more urgent care is needed, and parents can use the same checklist at home to judge whether their child is getting better or worse.