Perhaps people would be more likely to give the correct answer if "at least one of them is a girl" is rephrased as the equivalent "the youngest is a girl or the oldest is a girl".

Well constructing a different question to remove the “trick” of the problem is one approach. Kind of the “no child left behind” approach to riddles.

Another way is to say: the probability is 1/3, what is the probability? In this way, more people can answer it correctly, though perhaps not all.

Maybe people would be more likely to give the correct answer if the problem had _one_ correct answer. :-)