Having grown up in the Arizona desert, with no A/C for most of my life, I'm an expert on this subject. Loose fitting cotton (or other organic fibers) clothes (RE Lawrence of Arabia) and plenty of fluids. If you have a fan all the better, and if it gets too bad, wet the little tykes head and let the fan cool his head.
Reasons:
Nylon, polyester, and other synthetic fabrics are heat sinks and actually absorb heat from the air. That's why airline strewardesses greatest injury in an airplane crash is burns to the legs.
Natural fibers absorb the moisture from persperation and allow it to evaporate evenly providing natural cooling.
The head dissipates more heat than any part of the body and wet hair will cause the most effective cooling providing there is air flow.
I camp in the Arizona desert in the summer, with a 7 an 9 year old to teach them summer survival and we haven't had a bad time yet.
Loose cotton clothes and Hydrate, Hydrate, Hydrate
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