Children need to drink more water than adults. The effects of dehydration on a child's ability to think has been well studied and researched.

Research shows that dehydration is a common occurrence and even mild dehydration can impact a child's mental functioning. The good news is when given just a cup of water (300ml), a child's memory recall is significantly improved.

Another study showed that children who drank additional water performed better on attention tests compared to those who didn't.

So drinking water can have a positive impact on your child's mental performance and wile we can't make any definitive claims on increasing IQ it seems that like a good health measure to ensure your child is adequately hydrated as developing healthy hydration habits while children are young is a smart thing to do!

How Much Water Should Your Kids Be Drinking?

The Institute of Medicine of the National Academics has the following recommendation for kids:

Infants: breast milk is enough
Children 1-3 yrs: 4 cups per day
Boys and girls 4-8 yrs: 5 cups per day
Girls 9-13 yrs: 7 cups per day
Boys 9-13 yrs: 8 cups per day
Girls 14-18 yrs: 8 cups per day
Boys 14-18 yrs: 11 cups per day

(I cup = 250ml = 8 fluid ounces)

Tips to Encourage Kids to Drink More Water:

Total water intake comes from all sources: drinking water, beverages, food with lots of water content. Offer fruits and veggies with a high water content such as apples, cabbage, cantaloupe, cherry tomatoes, celery, cucumbers, grapes, pears, sweet peppers, watermelon, zucchini. A side of soup with meals can be an easy way to increase water count.
Encourage your children to drink water regularly throughout the day and be a good example by drinking regularly yourself!
Use fun drinking straws, special drinking cups and colorful water bottles. Stainless steel or BPA free are the best containers.
During physical activity, kids need more water, especially in hot weather. Encourage another 1/2 cup for every 20mins of activity.
It's helpful for kids to make drinking water a part of their daily routine. Encourage 1/2 to a full glass on waking, offer water with meals, snacks, and when they arrive from school, and again before they brush their teeth.
If your child says they aren't thirsty, encourage them to have a few sips of water. Generally this will encourage them to drink more on their own. Younger kids may need to try water about a dozen times over a two week period before they start to develop a taste for it. You can infuse water with sliced lemons, limes, berries, cucumbers or sprigs of fresh mint. For older kids, you can freeze berries or sliced fruit in ice-cubes, and add these to water for a hint of flavor.
Consider a reward system to motivate younger kids to drink such as posting stickers on a daily water log which can help (and you) track their consumption amounts. Even consider doing something special to celebrate their new healthy habit, like a trip to the water park or lake perhaps?