I don't go for hikes when it is that hot, period. But I always carry plenty of water in my car, hydrate like crazy before hiking and maintain hydration while hiking. Knowing how much water you sweat and respirate out of your body, you need much more than normal in arid climates.
The other place you find yourself in danger of dehydration is in cold climates, you never notice that you are dehydrating.
A good rule of thumb, if you are not 100% certain of resupply, you turn around at the point you have exhausted half your supply, go back to your starting point where you have extra reserves. Better safe than sorry.
That still leaves a lot of wiggle room for a person to get in trouble. Hyperthermia is just as dangerous as hypothermia and appears to be much more sudden in onset.
All in all, a huge tragedy. My heart pours out for the family, it is such a terrible shame to have a young life snuffed out. Even wholesome, healthy family outings can turn deadly in certain conditions.