It doesn't matter which child it is. Having a child is an independent action. There is no implied questioning like: if a couple is starting from scratch, what will the probability be, etc... As for the link to the HS girl's example, that really wasn't very impressive evidence. I will only believe this being 1/3 if someone can do what bonanova has requested and make a program. My answer is 1/2 unless I see that it is not via some kind of substitution example. Also, text books have been proven wrong plenty of times which is why they have multiple editions, so I'm not going to say: "oh, some random girl's High School Math Textbook says it's 1/3 so that must be the answer."...