My kids are fighting.

They're not my kids. I should stop personifying them.

My cat is being mean to my son.

Sorry, my cat is being aggressive with my dog.

As any parent knows, it's heartbreaking. It's keeping me up at night.

Literally. This morning at 2:45 a.m., a cat yowl woke me up. I had to take Pip the dog into the bed for protective custody.

It wasn't always like this. A year and a half ago, I borrowed Mimi from my mother to catch a mouse in my apartment. But what was supposed to be a month-long stay became permanent after I fell in love with this sweet little tuxedo cat.

Now she's just little and tuxedo.

Her bad cattitude came out of nowhere. My cat and dog used to get along perfectly. They cuddled together on my bed, he gave her space, and she'd occasionally rub up against him.

Basically, they had my ideal relationship.

Until a month ago, when she started attacking him.

Can a cat have a mid-life crisis?

Menopaws?

(That pun deserves hissing.)

Her mood swings come without warning. Pip will be minding his own business, padding toward the kitchen in hopes I'll feed him breakfast twice, when suddenly, Mimi will dart after him, chase him into a corner, and go full-blown cat-ninja on him, swiping the air, caterwauling and hissing.

Poor Pip never retaliates, he just scampers behind my legs like a frightened toddler, while Mimi saunters away.

She feels about as much empathy as Robert Durst exhibits.

I took Mimi to the vet, but she's perfectly healthy. The vet suggested she might be bored.

I didn't take offense.

But I've bought Mimi tons of toys since I got her, and the only game I've ever seen her enjoy is chasing after Pip's leash when it drags behind him.

I hoped this was more about the dragging leash and less about tormenting Pip.

So I got crafty and made a custom lure out of ribbon for Mimi's cat teaser, a fishing-pole-type toy. She seems entertained, but I think I'm more into it than she is. In an effort to tire her out, I find myself doing a full gymnastic ribbon-dancing routine.

I thought I was pretty good, too, but then my neighbors across the street held up two fives and a three.

Despite the increased playtime, Mimi still finds energy to lash out at the dog. So I decided that instead of winding her up, I needed to help her unwind.

With recreational drugs.

I have dime bags of catnip stashed all over the house. Anything to keep it mellow when the cat's eyes look a little too focused.

We're not casual users anymore. I'm now growing "cosmic cat grass" on my windowsill, which is semi-legal in New York.

It's medicinal, OK?

But Mimi has the tolerance of Seth Rogen, because the vibes remain harsh.

Pip is normally calm and submissive, but now I see how anxious he's become, especially when the cat is nearby. Last week, I came home and found what looked like a scratch wound on his back.

The worst part is, this drama has manifested a secret fear I have about my ability to mother. I've always imagined myself having a child, specifically one child. I was an only child myself, but secretly, I have another reason:

I'm afraid I'll play favorites.

How can you not? But maybe it's the way I'm wired. I'm very loyal, I love fiercely, and my brain naturally categorizes things.

For instance, Mimi is my beloved cat.

But Pip is my baby.

My mom said she'd be happy to take Mimi back, but I can't give up on her. I do love her, and there was peace between them for so long, I have to think we can get back there.

And this morning, after another rough night, when I was browsing cat behavior books online and feeling hopeless, Mimi leapt into my lap and began to purr.

She has a favorite, too.

Look for Lisa Scottoline and Francesca Serritella's most recent humor collection, "Have a Nice Guilt Trip," in stores now. Also look for Lisa Scottoline's most recent novel, "Betrayed," and her new book coming April 14, "Every Fifteen Minutes."