Actually, taking a second look, perhaps you are right. I originally connected the "lend your aid" with their care for the infant Jesus. Looking at it again, the "lend your aid" seems to be in connection with "While our hearts in love we raise", suggesting that they are assisting us in praising God for this act. I suppose it could be some "poetic license" in calling upon the church triumphant to join in with us in our praise of Christ, but perhaps it is more influenced by the expanding role of the "communion of the saints" throughout medieval theology. Any idea when it was written?

Interesting question.

~Jason