"But also, if someone volunteers that one out of their two children is a girl, then it's effectively-100% that the other is a boy, in real life." Well, yes, be

"But also, if someone volunteers that one out of their two children is a girl, then it's effectively-100% that the other is a boy, in real life." Well, yes, because saying one is implies that the other isn't, in speech. But we could talk about sets of two of anything that has a 50% probability.