Water is the best liquid for drinking at any age. Even infants may need some water, especially in hot weather. Children should always be allowed to drink as much water as they want, (this does not apply to soft drinks etc). Water does not flush any nutrients out of the body, but helps to keep the kidney working efficiently.
If you are concerned about the quality of your tap water use a Ultra Violet unit to destroy any bacteria in the water or boil the water before use and use a jug filter to remove any traces of heavy metals.