How tricky it was?
If you think that the Probability is 1/2 after reading that the couple has equal chance of having child of either sex then you are in wrong direction.
Take a look at the table below.
There are 27 possible combinations when boy is born on Tuesday. Out of which there are only 13 possible combinations where either boy (first or second) is born on Tuesday.
Hence the probability that the person having at least 1 boy off his 2 boys born on Tuesday is 13/27.