For the two-child family, the possible (first, second) outcomes are (BG, GB, GG, BB). Note that the question does not say that the boy you know about is the elder, just that (at least*) one child is male. Order does not matter.

Therefore the last outcome, (G, G) is not possible. The remaining first three outcomes are possible, but only one results in two boys. Therefore the answer is 1/3,

So, if you find out the sex of one child in a two-child family, you can guess that the other child is of the opposite sex and have a better than even chance of being correct.

* Thanks, David