When delivering a low blow, timing is everything.

You can greatly enhance the effect by acting like nothing happened when the other person cries out.  If the person makes a scene, tell him to stop whining.

The opposite is also true:  I once almost took out an eye of my friends’ four-year old, horsing around at the dinner table.  I hoisted him into the air, from a seated position, and he howled in delight and squirmed in the air, until I lost my grip on him, he went eyeball first into the back of the wooden chair and began howling in agony.  

I was immediately on the verge of tears myself, as I leaned anxiously over him, apologizing profusely.   He bawled for a moment, then saw my distress and I watched him pull himself together, rather quickly.   He stopped crying and told me he was OK.  He was reassuring me.

Damnedest thing I’ve ever seen, and one of the most beautiful.