Mary's "slow, smiling air" can be interpreted as an indication of how she feels about her life with Patrick. She's a loving, dutiful housewife, about to give birth to her husband's child. She's happy inside and happy at what's inside: her unborn baby kicking inside her womb. Her whole demeanor is an expression of the domestic bliss which she believes herself to enjoy. That this impression is profoundly mistaken merely serves to highlight Mary's innocence and unworldliness. Her insouciant air, almost child-like in its simplicity, provides a shocking contrast to how Mary conducts herself after she kills Patrick: methodically going about covering-up the scene of the crime.

Mary is flushed with the glow of life; both of herself and of her unborn child. Ironically, both her new life and the life of that child will occur after the end of Patrick’s own life upon this earth, and with it, the end of what Mary thought was her perfect marriage.

That particular line of text is meant to further highlight Mary Maloney's pregnancy.  It is often said about women who are pregnant that they have a particular "glow" about them.  The glow has been attributed to skin clarity, to a brightness of the eyes, and to a general overall happy demeanor.  Whatever the cause of the pregnant "glow," Dahl is calling attention to it with the line in your question.  Everywhere that Mary goes there seems to be a light happiness about her and everything that she does.    It even affects the very air that surrounds her.  A reader doesn't know that Mary is pregnant, when the reader reads the line in your question, but Mary is six months pregnant.  That detail is given in the next line along with several other pregnancy "glow" descriptions.  

Her skin -for this was her sixth month with child-had acquired a wonderful translucent quality, the mouth was soft, and the eyes, with their new placid look, seemed larger darker than before.