Thinking about the film almost 50 years later, it seems to me that Barnes' performance embodies the film itself. His wide-eyed innocence matches the nostalgic simplicity of this low-budget, film. Not only was it not shot in a sumptuous Hollywood studio, but it was shot in Britain, and rural northern Britain at that. The rugged Pennine landscapes may remain to this day, but the make up of the farms and villages are long gone. Likewise, it's hard to imagine this story happening today. Barnes would most likely be shut inside playing on his Wii, not allowed to go to the barn alone in case he ran into, well, an unsavoury character hiding therein. And doubtless Bates would be far too scared of being caught and branded a paedophile to hang around enjoying the children's hospitality.
Even were such a movie to be made today, it would be difficult for the filmmakers to avoid an uncomfortable, suspicious atmosphere in showing a relationship between a young boy and a unrelated, hitherto unknown man. You only have to look at how little evidence the makers of Doubt required to perfectly balance the "did he / didn't he" question that dominated post-viewing discussions.
Anyway, Ron's conclusion is particularly worth quoting:
But the world of the film is unique, and for all its simplicity it grows richer on repeat viewings. Perhaps the film escapes the expected coyness because we are constantly aware that the man in the barn is not Jesus – he may even be an escaped criminal – and the convolutions the children indulge to sustain belief take the piss out of adult "true believers." At the same time their simple loving concern for this man has an undeniable sacredness about it. There's a thin line between serving Jesus and visiting a criminal, whether in a stable or in a prison.