I have to agree......I worked in the Base Exchange at Castle AFB in the late 60s......and I’m an AF Brat......I never heard of such a thing.....plus my mother was an asst manager at the Commissary at the same time......something’s not quite right here!
I don’t know what part of the Exchange you worked in, but when I came on active duty in 1971, officers were prohibited from wearing Levis or other sorts of blue jeans to the base exchange (AAFES) at a Strategic Air Command base. Enlisted, yes, but the jeans had to be clean and “presentable.” Nobody would have been allowed to wear workout clothes.
I am still amazed, though, that the Commissary (not part of AAFES) would have a strict dress code for kids. If true, it is probably something the local command came up with.