Michaelr: fair question.

Child identifies 306 ballads. But he prints many more than 306 texts because a traditional ballad comes in more than one version.

Since "The English and Scottish Popular Ballads" was published in 1898, thousands more versions of something like two thirds of Child's 306 ballads have been collected.

Taylor found new evidence to suggest that a version of "Sir Aldnigar," unknown to Child, was sung in the twelfth century. If true, that would make "Sir Aldnigar" the oldest known "Child ballad."

The one known version of "Judas" is the oldest piece in Child's collection. But it is not necessarily the oldest "Child ballad."