Paul is correct - the word "Child" or a letter "C" was hand-stamped in red on the ticket by the issuing clerk.
Steamybrian is close with the alternative - the adult ticket was cut, but not diagonally from bottom left to top right, but at a more gentle angle from a position just to the left of the centre-line along the bottom to just to the right of the centre-line on the top, thus distinguishing it from the normal vertical cut dividing the ticket into outward an return portions.
As such you then had two tickets bearing the same number, and, as a child I used to spend my hard-earned pocket money on a Saturday trip, only to find on the following Saturday I was sold the other half of the same ticket!
So, over to Paul for the next one.