My six-year-old daughter told us at dinner the other day that her teacher generally allows her class certain privileges "if we be good." The first thing that struck me is that it sounds like the subjunctive. An example would be If we be faithful to Christ, he will certainly be faithful to us from the New American Standard Bible. This seems extremely unlikely since it's an archaic form, although you can see how it would have come about. Other verbs appear in the plain present tense here, which is identical in shape to the subjunctive; both use the base form; only be has a distinct first-person present-tense form. This is probably, then, a performance error stemming from mistaken analogy.
The second was that if we be good gives be a dynamic sense that if we are good simply wouldn't have. It sounds much better than, for example, if we be hungry. Consider that you can say we're being good where you can't say *we're being hungry (unless this is some kind of dramatization of hunger).
After we adults had discussed the grammatical implications for a few minutes, my daughter said, "Oh, I mean if we are good," which I found somehow both wonderful and regrettable.