Who was the second child of Henry the eighth?

It really depends is you are only counting his three legitimate children, Mary, Elizabeth and Edward. If this is what you are looking for, his third child is Edward, the next Tudor king of England.

However, he undoubtedly fathered many illegitimate children (one known, Henry Fitzroy, Duke of Somerset) and legitimate children that died within a few months of being born.

If you count all of his known children that lived, his third child is Elizabeth, as Henry was born before Mary.