Billy looked at the following guidelines to summarize the approach to a child with gastroenteritis: 

UK NICE Guideline: Diarrhoea and vomiting in children under 5 (Issued: April 2009) http://guidance.nice.org.uk/cg84
CPS Guideline: Oral rehydration therapy and early refeeding in the management of childhood gastroenteritis (Posted: Nov 1, 2006) http://www.cps.ca/documents/position/oral-rehydration-therapy

History:

onset of diarrhea and/or vomiting (gastro is sudden in onset)
duration of vomiting and diarrhea (diarrhea 5-7 days, max 2 weeks; vomiting 1-2 days, max 3 days)
sick contact
pathogen exposure
travel history

History suggestive of increased risk of dehydration:

young age (esp 5 diarrhea in 24h
>2 vomiting in 24h
no oral intake
signs of malnutrition

Think about differential diagnosis if:

fever >38 in children younger than 3 months
fever >39 in children older than 3 months (fever workup required)
shortness of breath or tachypnoea
altered conscious state
neck stiffness
bulging fontanelle in infants
non-blanching rash
blood and/or mucus in stool
bilious (green) vomit
severe or localised abdominal pain
abdominal distension or rebound tenderness.

 
SSx of dehydration and shock
Table 1 in NICE



Increasing severity of dehydration

No clinically detectable dehydration

Clinical dehydration

Clinical shock

Symptoms (remote and face-to-face assessments)

Appears well

Red flag Appears to be unwell or deteriorating

–

Alert and responsive

Red flag Altered responsiveness (for example, irritable, lethargic)

Decreased level of consciousness

Normal urine output

Decreased urine output

–

Skin colour unchanged

Skin colour unchanged

Pale or mottled skin

Warm extremities

Warm extremities

Cold extremities

Signs (face-to-face assessments)

Eyes not sunken

Red flag Sunken eyes

–

Moist mucous membranes (except after a drink)

Dry mucous membranes (except for 'mouth breather')

–

Normal heart rate

Red flag Tachycardia

Tachycardia

Normal breathing pattern

Red flag Tachypnoea

Tachypnoea

Normal peripheral pulses

Normal peripheral pulses

Weak peripheral pulses

Normal capillary refill time

Normal capillary refill time

Prolonged capillary refill time

Normal skin turgor

Red flag Reduced skin turgor

–

Normal blood pressure

Normal blood pressure

Hypotension (decompensated shock)

Table 2 in CPS



TABLE 2
Clinical assessment of degree of dehydration *

Mild (under 5%)

Moderate (5-10%)

Severe (over 10%)

Slightly decreased urine output
Slightly increased thirst
Slightly dry mucous membrane
Slightly elevated heart rate

Decreased urine output
Moderately increased thirst
Dry mucous membrane
Elevated heart rate
Decreased skin turgor
Sunken eyes
Sunken anterior fontanelle

Markedly decreased or absent urine output
Greatly increased thirst
Very dry mucous membrane
Greatly elevated heart rate
Decreased skin turgor
Very sunken eyes
Very sunken anterior fontanelles
Lethargy
Cold extremities
Hypotension
Coma

*Some of these signs may not be present

 
SSx of hypernatremic dehydration:

jittery
increased muscle tone
hyperreflexia
convulsions
drowsiness or coma

Labs:

No routine blood work
Serum sodium, potassium, urea, creatinine, glucose if IV fluids or signs of hypernatremia
Blood gas if shock suspected

Stool culture if:

blood and/or mucus in stool
immunocompromized
septicemia suspected
travel history
diarrhea not improved by day 7
uncertainty about diagnosis of gastroenteritis