There's no magic number (did you know that "8 glasses a day" stuff for grownups has no scientific basis? and that ANY liquid you drink counts, even if it has caffeine or sugar in it?).

Make sure he or she gets plenty of liquid with meals, and that if he or she asks, it's available.

Water is the second most important nutrient for life, after oxygen. More than two-thirds of the body is made up of water, with the brain being made up of nearly 85 percent water and solid tissue (like muscle) being made up of about 70 percent. Are we drinking enough? Not by a long shot, according to a survey released on May 11, 1998, from the Nutrition Information Center at the New York Hospital-Cornell Medical Center.
The survey shows that "a significant number of Americans may be drinking themselves to dehydration by consuming too little water and too many beverages that rob the body of water (alcohol,caffeine)."
While the survey of 3,003 respondents did reveal that the "average American" drinks nearly eight daily servings of hydrating beverages (such as water, milk, juice, and decaffeinated soft drinks), this is undermined by the nearly five servings of diuretic caffeine- or alcohol-containing beverages reported.
"The net result is that most Americans are probably only getting about a third of the valuable hydration benefits they need," says Barbara Levine, R.D., Ph.D., director of the Nutrition Information Center. "The vast majority aren't drinking enough water."
Water has an important role in nearly every major function in the body -- regulating body temperature, carrying nutrients and oxygen to cells, removing waste, cushioning joints, and protecting organs and tissues.
It's the major component of blood and contains much dissolved oxygen that is easily assimilated by the body; it helps prevent constipation; and it flushes the body of metabolic wastes.
Water also helps maintain muscle tone, supplies oxygen and nutrients to the cells, moves waste products and toxins out of the body, and acts as a natural "air conditioning" system. This clear liquid is also crucial for metabolic reactions. When the body becomes dehydrated, metabolism slows down, and electrolyte imbalances can occur, leading to a substantial decrease in energy levels and, over time, nervous disorders.
8 a day?
Remember that the general recommendation of eight glasses of water a day may vary widely depending on different factors, including body size, climate, and environment, as well as living habits and activity levels.
As you might guess, high levels of physical and mental activity, and excessive stress, increase the body's requirements for water. Water is also used for the breakdown of protein, so, if you increase your in-take of protein-rich foods, your level of water consumption should increase, as well. If you smoke cigarettes or drink alcohol, water is crucial in moving metabolic wastes through the body to lessen the burden on the kidneys and liver. In fact, water is called the universal solvent since it's capable of dissolving and carrying nearly any kind of matter, including toxins.
Incredible as it may seem, water is quite possibly the single most important catalyst in
losing weight and keeping it off. Although most of us take it for granted, water may be
the only true "magic potion" for permanent weight loss.
Water suppresses the appetite naturally and helps the body metabolize stored fat.
Studies have shown that a decrease in the water intake will cause fat deposits to increase, while
an increase in water intake can actually reduce fat deposits.
Here's why: The kidneys can't function properly without enough water. When they don't
work to capacity, some of their load is dumped onto the liver.
One of the liver's primary functions is to metabolize stored fat into usable energy for the body. But, if the liver has to do some of the kidney's work, it can't operate at full throttle. As a result, it metabolizes less fat, more fat remains stored in the body and weight loss stops. Drinking enough water is the best treatment for fluid retention. When the body get less water, it perceives this as a threat to survival and begins to hold on to every drop. Water is stored in extra-cellular spaces (outside the cells). This shows up as swollen feet,legs and hands. Don't substitute caffeinated coffees, teas, and sodas for water. Caffeine acts; as a diuretic, causing you to lose water through increased urination. Alcoholic beverages have a similar effects Diuretics force out stored water along with some essential nutrients. Again, the body perceives a threat and will replace the lost water at the first opportunity. Thus, the condition quickly returns. The best way to overcome the problem of water retention is to give your body what it needs .... plenty of water.
Only then will stored water be released. If you have a constant problem with water retention, excess salt may be to blame. Your body will tolerate sodium only in a certain concentration. The more salt you eat, the more water your system retains to dilute it. But getting rid of unneeded salt is easy just drink more water. As it's forced through the kidneys, it takes away excess sodium.
The overweight person needs more water than thin ones. Larger people have larger metabolic loads. Since we know that water is the key to fat metabolism, it follows that the overweight person needs more water.
Water helps to maintain proper muscle tone by giving muscles their natural ability to contract and by preventing dehydration. It also helps to prevent the sagging skin that
usually follows weight loss...shrinking cells are buoyed by water which plumps the skin and it
clear, healthy and resilient.
Water can help relieve constipation. When the body gets too little water, it siphons what it needs from internal sources. The colon is one primary source. Result? Constipation. But, when a person drinks enough water , normal bowel function usually returns.
So far, we've discovered some remarkable truths about water and weight loss:
*The body will not function properly without enough water and can't metabolize stored fat efficiently.
*Retained water shows up as excess weight.
*To get rid of excess water you must drink more water.
*Drinking water is essential to weight loss.
How much water is enough? On the average a person should drink eight 8oz. glasses every day. That's about 2 quarts. However, the overweight person needs one additional glass
for every 25 pounds of excess weight. The amount you drink also should be increased if you exercise briskly or if the weather is hot and dry. Water should preferable be cold...it's absorbed into the system more quickly than warm water. And some evidence suggests that drinking cold water can actually help burn calories.
To utilize water most efficiently during weight loss, follow this schedule:
MORNING--1 quart consumed over a 30min. period.
Noon-- 1 quart consumed over a 30min. period.
Evening--1 quart consumed over a 30min. period.
When the body gets the water it needs to function optimally, it's fluids are perfectly balanced. When this happens you have reached the "breakthrough point".
What does this mean?
*Endocrine-glad function improves.
*Fluid retention is alleviated as stored water is lost.
*More fat is used as fuel because the liver is free to metabolize stored fat.
*Natural thirst returns.
*There is a loss of hunger almost overnight.
If you stop drinking enough water, your body fluids will be thrown off balance again and you may experience fluid retention, unexplained weight gain and loss of thirst. To remedy the situation you'll have to go back and drink more water.

Just some feedback. Caffeinated beverages do not count, because they are a natural diuretic, tending to actually make you lose the water (or juice/milk/etc) that you already drank.

As far as how much: Is she potty trained? Check the potty. The urine should be light yellow. If it is dark yellow, they are not drinking enough. (same holds true for adults).