If there were two possible statements they asked

"a random family has been sampled, the sample family has two childs, one of them is a girl"?

and

"a random family has been sampled, the sample family has two childs, one of them is a boy"?

and they selected each statement based on randomly picking a child from a random family then the probability actually becomes 50% boy/girl for the next child since the boy/boy or girl/girl has twice the chance of generating the above statement for the respective gender compared to the mixed gender children family.

Ie. if they say one is a girl that statement had a 50% chance of being generated by a girl/girl family (since we pick the statement based on a random selection of one of the two childrens gender and there's 2 girls, doubling the chance of a statement that one's a girl coming from a girl/girl family), there's 25% chance the statement was generated from a girl/boy family and a 25% chance the statement was generated from a boy/girl family.

If you take 50% chance girl/girl, 25% chance boy/girl and 25% girl/boy you'll see there's a 50/50 chance of the next child being either gender.

All this due to changing how we sampled.