As a general rule, unless you are competing at the level of Olympic athletes water is all you really need. The gatorade and powerade drinks are nice but have too much sugar to help.
The hydralite type drinks are overkill for most of the population. Those with extreme health issues and those who compete at Olympic/professional levels need those things but very few ever reach those levels of activity.
There ARE things you can do to make water more attractive without making it as expensive as the hydralite drinks or as sugary as the gatorade drinks.
While water CAN kill you, so can everything else. Using that woman's death in this situation is not very applicable. She was not drinking because of an activity or because she was thirsty.
As a general rule by the time you are thirsty it is long past the time to drink.
My father in law coached many state championship football teams at the high school level. He paid very close attention to what the latest research was saying. I also had a grandparent who worked with MANY researchers who examined this topic in the collegiate and pro sports levels. He was a statistician with an encyclopedic memory.
If you want to use gatorade, it is best to get the powdered kind and use 1/3 to 1/4 the powder recommended for an amount of water. If it says to use 1 scoop for 32 oz water, use 1 scoop for 96 to 128 oz. It will not taste the same, but will provide enough of what they need.
An even better solution is to soak a pound of raisins in water for a day or two. (If soaking for longer keep refrigerated). Add the liquid to a gallon of water and it will really help.
Adding lemon or lime or even orange or grapefruit juice to water will help it taste better and help your body with the demands of sporting activities. You can get powdered lime,lemon and orange juice under the brands TrueLemon, TrueLime, and TrueOrange. (
http://www.truelemon.com/ you can even click for samples or buy online). This is an excellent product and makes adding lemon or lime to a drink very easy. No more lemon juice packets that break open and leak all over everything.
The homemade sports drinks such as the recipe posted by Mandy are also probably overkill for a child but are not as terrible for them as gatorade or powerade. (because of all the sugar you might as well give your child a soda - they truly are not a good idea during sports).
This may sound low tech or like I am poo-poo ing all over others' suggestions. I don't mean to. It comes from years of listening to researchers and coaches and people getting degrees in nutrition and sports medicine and athletics talk about this. I even helped enter data for several studies on this (a part time high school and college gig for a faculty brat, LOL!).
Whatever you choose, make sure your child drinks enough in the hours before and after the event or outing as well as while he is playing. MANY headaches are caused by dehydration. We also eat far more than we should because many of us perceive thirst as hunger. So if you can drink a glass of water when you are hungry you may be able to cut your calories painlessly!