I used to get that when I worked at a grocery store. The kids would stare and stare. I would usually scowl right back at them and they'd go flying under their mother's skirt *LOL* The other time this kid was behaving so bad, I mean out of control. The mother turned her back so I reached over and growled 'you'd better shut up or I'll box your ears'. That kid was quiet the whole time hehehehe.
I sorta like the low-spoken threat. That way, you still have plausible deniability if the kid accuses you of saying something mean to him. "I don't know what he's talking about. I didn't say anything. I was clearing my throat."
You're paying attention to this, right, Petronski? ;-)