In our house, "Leave it" can apply to cats or kids equally. They act much the same, lying in wait to torture the unsuspecting other, butts wiggling in delight (that only applies to cats), then pouncing hard to elicit snarls and hisses from the target. Jack** was particularly guilty of this yesterday, when poor Rex had just had a bath and was angry at the world, slinking around and shaking his wet legs in irritation. Aden and I warned Jack to stay away, that Rex was aggrieved and ready to snap, but Jack fixated all the more on his rival, earning boxed ears and scratches for his pains.
That happens with the kids, too. When William was ticked off at the world two days ago because he couldn't manage a standing back flip, I warned Daniel to stay away. Did he listen? Absolutely not - Daniel made a beeline for his aggravated sibling and I had to break them up before the verbal slings and arrows turned to blows. Correct me if I'm wrong, but don't novels say things like, "He knew well enough to stay away when his brother was angry, wary of the fists and fury." I feel like I've read such statements many times before, but rarely seen such avoidance in real life. I've rarely read the words "Leave it!" either, but that has become my most useful tool, likely to appear anywhere from blog entries to masters' theses.
**Jack is a somewhat foolish feline, recently earning the nickname 'Ryan Lochte of cats.'