Not real sure what the answer is, but we have found with our kids, that we keep water bottles in easy reach. For my 4 year old, we have a bike basket on her trail-a-bike, with a water bottle tethered inside. Even without us stopping, she is able to quickly grab the bottle and get a drink. The tether keeps her from dropping it and thus causing an accident.
For our 18 month old, who rides on in a bike seat, we do the same setup. Sippy cup, tethered to the belt that secures him in. If he gets thirsty, he just grabs it and gets a drink. Again we don't have to worry about it falling and causing us to have an accident.
For ourselves we have never found that we needed to worry about our kids dehydrating. They always wear nice sun hats, have plenty of water, and in really hot weather, we put cold hankerchiefs on there necks to keep them cool. I guess that we just follow reasonable precautions, start early in the mornings to avoid the hottest part of the day, and look for more shady places to ride in the warmest part of the day.
I figure that if I am thirsty, the kids needed a drink 20 minutes before! Good luck with the research!