You mean in the epilogue, before the extras? No, he didn't tell him. He send him pictures, and Yeonwoo assumed that one of them was a girl, which wasn't true.
But in that epilogue Josh was pregnant with his third child (only mentioned in the novel) which later turned out to be a girl, but her name is still unknown and she didn't appear so far.
And Cecil wearing cute/girl's clothes, why it happened etc. is a side story that wasn't adapted to manhwa so far.

Wait just curious, but Pete and Cecil, they're both boys? Then why did Josh tell Yeonwoo he had a boy and a girl?