Jinsterrr, there is a difference. For the first question, the probability of the missing child being a girl is 75%. Because here there is no expanded state. There are simply four choices: BBB, BBG, BGB, or GBB. Three of the four include one girl, so the chance is 75%. The simulation also demonstrates this.
But when you say that a 3C2 yielded two boys, you have more information. BBB is guaranteed to result in two boys. BBG, BGB, or GBB are have a 2/3 chance of introducing one boy and one girl. Just as DrPizza stated, this is when you look at the extended state.
But when you say that a 3C2 yielded two boys, you have more information. BBB is guaranteed to result in two boys. BBG, BGB, or GBB are have a 2/3 chance of introducing one boy and one girl. Just as DrPizza stated, this is when you look at the extended state.