The probability of each gender is 1/2, so that:
[tex]P(\text{girl)}=P(\text{boy)}=\frac{1}{2}[/tex]The gender of the second child is independent of the gender of the first child, this means that the gender of the first child does not modify the probability of the second child being born a girl.
So the probability that the second child is a girl is equal to the probability of the baby having said gender:
[tex]P(2\text{ndGirl)}=\frac{1}{2}[/tex]