I know you are, but what am I?

so the kids were acting owly today -- taunting each other and being so picky with each other that at last I lowered the boom.
"Be quiet, can't you?" I said, too loudly.
this caused them to lower their heads and scuff their feet on the floor as they wandered off in different directions. but a half hour later they were at it again. this time in the backyard. what kind of sick magnetism draws kids together when clearly they hate each other's guts? I decided to be positive and proactive with them, as their mother would be if she were around. so I poured sugar into a plastic bowl and took it out to them.
"Here" I said kindly, "go pull some rhubarb stalks, clean 'em off with the hose, and dip 'em in sugar. they taste wonderful. I used to do that as a kid. Oh, and don't eat the leaves -- they'll make you sick." 
it worked for about an hour; they sat companionably together in the shade of the apricot tree munching on reddish green rhubarb stalks. and then they went back to calling each other names like 'dopey eyes' and 'flush bucket.'  They kept saying "I know you are but what am I?" when the hitting started I sent them all to their rooms and threatened them with kale soup for dinner.
"It's very healthy" I told them them in a low and silken sinister voice.