I don't have official answer.
But, here is my explanation.
Total possibilities = BB, GG, BG, GB
One(Atleast) is BOY = BB, BG, GB
Another is Girl,given than other is BOY = BG , GB
P = 2/3.
As i said before, I don't have official answer!
The question should be worded "at least one is a boy" and in that case, 2/3 is the answer. Trust me.
However, as worded, the question states: "ONE is a boy". hence the other child MUST be a girl. If the other was a boy, then the statement "ONE is a boy" would not be true -- because in that case TWO of them are boys!
Of course, this is an oversight and the GMAT would not be so ambiguous.
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993