It does not really matter.
In the past I have quoted the same Karl Barth reference, and I've also heard it similarly attributed to Lord Kelvin (William Thomson). The story went that Lord Kelvin was a being asked of all his great discoveries and investigations ... what was the greatest thing he ever learned?
Whether or not the Karl Barth and/or Lord Kelvin stories are legends or real, is not all that important. The words of that 'child's song' still ring true:
Jesus loves me, this I know, for the Bible tells me so ...