With the given information, the P(boy) for any child is 1/2. Extra info could have been given to show that the P(boy) or P(girl) would be more or less than 50%. But absent info we have to assume 50% for each.
These events are independent. In other words, having a boy the first time doesn't change the probability of a boy the second time. P(A AND B) = P(A)*P(B).
P(BOY and BOY) = 1/2 * 1/2 = 1/4 = 25%
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