Basically "darndest" only exists as part of the expression "kids say the darndest things," which just means that small children are always saying surprising, unexpected, funny, and clever things. I don't think I've ever heard anyone use the word "darndest" in any other context besides "X say the darndest things." It is meant to be the superlative of "darned," which is a polite way of saying "damned."

Kids say the craziest things.

Gentlemen! Thanks everyone for all your answers! I understand now and they were all very helpful.