Some symptoms are signs that the child is so affected by its illness that you must contact a doctor immediately.
Drowsy/apathetic: The child will for example only lie in bed and does not want to play.
Does not drink or pee: Most children do not want to eat but can be lured into drinking. If the child does not drink it will become dehydrated and urination will cease.
Irritable: The child reacts irritably towards light and sound.
Cadaverous and pale: The child’s face looks unhealthy.
Strained breathing: Can be a sign of pneumonia
Unable to put forehead against its knees or look at its navel: This can be caused by a stiff neck and back and might be a sign of meningitis.
Dot-shaped red spots in the skin: The spots do not disappear if you press a spectacle lens against the spots and look trough the glass. It can be a sign of meningitis.
Further reading on Good signs