If this is a real racing kart, then sheet metal sides are out of the question as every kart sanctioning body doesn't allow sheet metal to be used as such.

A better solution is to use regular kart molded plastic side pods and fab up 5/8"-3/4" tube mount system. The molded side pods look good, are quite inexpensive and will do way more to protect your sons legs.

Gord

Here is a link to a handy chart that lists gage thickness and pounds per square foot. It might be handy if weight is a factor.

http://www.rollformedshapes.com/gagechart.htm

Here is the sheet metal gauges chart for your reference:
https://machinemfg.com/sheet-metal-gauge-sizes-chart-inch-mm/