What is the ruling on a husband shaking hands with his wife and kissing her cheek upon leaving or returning home? Is it permissible to do so in front of older children? And does it nullify ablution?

Kissing one's wife upon leaving or arriving, or at any other time, is not obligatory. Performing it in the street in front of people is not in keeping with good manners; in fact, it is impermissible. Similarly, it is not appropriate to kiss her in front of discerning children, as it contradicts modesty. Touching a woman does not invalidate ablution (wudu') at all, even if done with desire, unless something (an impurity) is emitted. Shaking her hand is permissible and there is no harm in it.