Bonny at Morn
I wish to venture another possible meaning of this wonderful song.
I interpret it to be part lamentation, part resignation, of a new father, about the disruption brought by an new infant in the family .
He attributes fault to himself and his wife- that she is pleasant and lovely, "Canny", at night and beautiful , "Bonny" in the morning. "Thou's ower lang in thee bed" (refers to them having sexual intercourse) producing the bairn.
He laments the disruption "Thou hinders thee mither " and "We're all laid idle wi' keeping the bairn"
He tells of his resignation with "The lad winnot work and the lass winnot learn" refers to either himself and his wife or young couples all. That is to say, despite the disruption caused by infants, lads and lasses will continue the activity which produces them.