The ballad form did not develop until about the 13th century. Thus there is no such thing as a "pre-Child" ballad. He included all items he thought were ballads, of any era. There may be some that escaped his attention, but not because they were too old.

There are several arthurian ballads in the Child collection, though I believe that they were never really ballads but metrical romances. The Boy and the Mantle, King Arthur and King Cornwall, The Marriage of Sir Gawain, etc.

Most of the early arthurian material is Welsh. Look for a good edition of the Mabinogion, as was suggested above. Look also for "The Spoils of Annwfn," which is sometimes but not always included in the Mabinogion.