This is a more common problem than one may think as children don't always recognize the early signs of thirst.
Many children don't drink enough water, and in this weather it is critical that kids be well hydrated. At birth, a baby’s body consists of about 75% fluid as they grow older (and this is also true for adults), approximately 60% of the body is fluid. We need water to effectively run most physiological processes, and also to get rid of waste products.
Infants and young children don't tolerate heat well so drinking small sips of water regularly is essential when they are outdoors. Even slight dehydration can make children irritable and contribute to tiredness, headaches, dry skin, poor concentration and mental performance.
The amount a child needs to drink can vary depending on age and physical activity. Here is a general guideline. If kids are active especially outdoors, add more water. Also when kids are unwell its best to increase fluids.
4-8 years need 1-1.5L per day
9-13 years need 1.5 to 1.7L per day