"He's whipped!" Harmon, 36, told PEOPLE about Sehorn, 38, at the Dior Beauty Luncheon on Wednesday afternoon at the Shangri-La Hotel in Santa Monica. "He was pretty good with the first two. He was like, 'I got this, I'm totally in control.' The third one, she's thrown him for a complete loop. It's the best!"
Harmon gave birth to daughter Emery in December, the third child for her and the former NFL player, whose other daughters are Finley, 5, and Avery, 3.
"He's an amazing, amazing father and they all clamor for him," said the actress. "Now [Emery] can sit up and she can hold herself up so she just props herself up on his arm, puts one arm around his shoulder and just walks around with him. She loves it."
When Dating StartsHarmon can't wait until the girls start dating – so she can really see their effect on Sehorn.
"My husband is so calm anyway. If the whole family were engulfed in flames he'd be like, 'Relax, I've got it.' I'm like, really? I can't wait til they start dating, because he's going to be running around the house freaking out, and I'm going to be like, 'Relax, I've got it. Just a second, please – we're going to go talk about their first kiss and their broken heart, and you stay out of the room. You go chase that boy down the street, because you know what's going on.' "
For now, with a newborn in the house, the couple is enjoying every waking minute.
"This is my last baby," said Harmon, "so I don't even have those sleepless nights anymore. Like, I'll never have that again in my life, where you're standing around, rocking with tears streaming down your face thinking, 'If I don't sleep, I don't know what I'm going to do, because I haven't slept in three days.' All that's behind me now, I never get to do that again."