Phil,
The ESPB itself is still the prevailing commentary on each of the 305 ballads. Yes, further versions have surfaced since his time and he was largely blissfully unaware that some of these ballads existed on his own doorstep. The first stopping point for all scholars is still usually what Child had to say about the ballad in his headnotes to each ballad. The next point might be Bronson or finding out if anything further has been done on a particular ballad. There are some glaring errors but these are simply because he didn't have the necessary information at that time. For instance, many of his notes to Child 20 actually apply to 21 but from the info he had he wasn't to know that.

Matt, yes most came from existing collections. Child was a scholar and editor, not a collector. Some came solely from broadsides (e.g. most of Robin Hood ballads) but not that many.