You didn't mention if your child was ill or not. The body weight seems on the low side. 3 - 6 cups of water per day for your child's weight. If your child has kidney problems be careful not to over-dose your child on too much water. If your child does not have kidney problems and wants water then let them have it (within reason.) It is best to spread out the intake of water for a child than giving them 3 - 6 cups all at once per day. Children can become thirsty just playing or active in sports. Juices also count as fluid intake. Here is some good advice off a medical website: **** It is possible for a person to drink too much water. It is called water intoxication. What happens is the sodium level in the blood reaches very low levels (because of dilution by excess water which can only be excreted in the urine, sweat or breath). This disturbs water balance in the brain, which can cause epileptic seizures and even death. ****Cold water is the best fluid to satisfy a thirst and the most effective to replace fluid lost through exercise and perspiration. Keep a pitcher of water in the refrigerator during summer months for thirsty children. ****You should be aware of how much fluid your children drink especially in summer. Your four-year-old can probably tell you when he's thirsty, but your 21-month-old child may not. Whenever your four-year-old wants a drink, offer some liquids to your 21-month-old also. Another guidelines for those parents with only one child is, whenever you get a drink for yourself, offer some to your child. This includes offering a bottle of water to an infant. By the time you are thirsty though, you are already somewhat dehydrated. So drink water every waking hour to anticipate thirst especially in hot weather and remember to offer your children water every time you do. ****At birth, 75% of the body weight of a child is water. This decreases to approximately 60% by age 10. To put this on a practical level, infants ages birth to two years (6 to 26 pounds), should have three to six cups of water per day. Children age's two to 12 years (26 to 100 pounds), should have four to eight cups of water per day. ****Since the amount of fluid required per day is determined by your body temperature, the ability of your kidneys to remove wastes and sweating, a second guideline would be helpful. When your child goes to the bathroom, his/her urine should be light yellow or colorless and odorless, unless it is the first urine after getting up in morning. Also check infants soiled diapers for urine color and odor.