Often a spider or scorpion, rarely snakes, will crawl into a warm boot or shoe to soak up some warmth. Not likely to see them during the daytime and your son should be fine.
In all the years I lived there, I found a spider once and a scorpion once. It's very common if you live in a new development as the soil has been recently turned up and they will be hunting new homes. A more established area isn't as likely. Either way, it is a habit you should get into because if you don't, you will find out too late. Not meant to scare you, just a part of living in the desert. The more the area is built up, the less likely it will ever happen. But why take chances?
BTW - the water tastes terrible.