In the case of two children, there are 4 equally likely possibilities
BB, BG, GB and GG;
where B = Boy and G = **** and the first letter denotes the first child.
From the question, we can exclude the first case of BB. Thus from the remaining 3 possibilities of BG, GB & BB, we have to find the probability of the case with two girls.
Thus, P(Having two girls given one ****) = 1 / 3