Nick and Rob have the right answer 1:2 50% though Nick's reasoning was not quite correct - will post post full answer later.

Is it not bizarre though;

If I tell you I have two children one of whom is a boy the odds of me having two boys is 1:3 33%, yet if I tell you I have two children one of whom is a boy who happens to also be my thinnest child the odds of me having two boys is 1:2 or 50%.

No tricks, no genetics, just maths!