It is often claimed that Childs' canonical approach rests on the presupposition that the Biblical Canon is a hermeneutically sealed, self-referential unit (e.g. Barr and Barton). To be honest, I'm not entirely sure how anyone could come to that conclusion on the basis of Childs' actual exegetical work, but even in theory Childs has explicitly rejected this misunderstanding. For Childs, the unity of the canon does not exist within a neat "narrative world" that has no connection to the complexities of extrinsic reality (rather like a fantasy novel which we can believe in while reading it but which has limited connection to extrinsic reality). In fact, precisely the opposite is the case! The unity of the canon consists precisely in its extra-canonical referent. This referent is God, and as such it is as complex a reality as one can image (if "complex" is the right word; it makes God sound like a puzzle to be solved ... ). Given the nature of this extra-textual reality, it is necessary that the full voice of the canon be brought to bear in trying to apprehend and respond to it (or Him) adequately. Here is Childs in his own words (in response to Barr's critique of his Introduction; the key phrase here is "the fullness of extrinsic reality"):
“I certainly confirm that Israel's faith was grounded in anterior reality. First in oral tradition and subsequently in written form Israel bore testimony to God's redemptive intervention on its behalf. These events of divine deliverance were not simply recorded, but continually re-interpreted throughout history. Israel actively shaped its traditions while at the same time being formed by the very material being transmitted.
Because of the peculiar nature of Israel's tradition which is reflected in the multi-layered testimony of the canonical text to this sacred history, there is no direct access to the fullness of that extrinsic reality on which the faith was grounded apart from Israel's own testimony. One important purpose of establishing a normative canon was to mark the special relationship of the community to these witnesses.
... The central point to be made is that the nature of Israel's testimony to historical events varies greatly and that extrinsic reality can be represented in innumerable ways ... .” (Childs, “Response,” 53, 56).