wear a straw hat to keep the sun off of your head.

wear light-colored, light-weight clothing.

you will have to carry with you, and drink, a lot of water.

consider 2 other things:

1. being small they will make less effort moving etc and so be less likely to overheat

2. being small their surface area to volume ratio will be higher thereby increasing the effectiveness of sweat for cooling hence sweat less

Keep in mind skinny people with very little muscles don't have much back of water in their muscles since they have little muscles to store back up alot of water. So how do skinny people survive being in the hottness all day?