Jessica Simpson Reveals 17-Month-Old Daughter Birdie's Sweet First Word: 'She's the Cutest'

Jessica Simpson is in awe of her trio of children.

The mother of three, 40, tells PEOPLE that her little ones — son Ace Knute, 7, and daughters Maxwell Drew, 8, and Birdie Mae, 17 months — whom she shares with husband Eric Johnson, are "growing up so fast."

Case in point? How her two older children behave with their younger sibling, who joined the family in March 2019.

"Oh, she is the cutest. Oh my gosh," Simpson says of Birdie. "And it's so cute to watch my older kids with her. My son and her are like BFFs! The cutest. When Birdie is laughing, like cackling, that is a contagious thing throughout the family. Her first word was 'Ace.' "

Simpson adds that her "very observant" youngest child "says hi to everybody."

"She'll stare you down for a bit and then, 'Hi. Hi.' She's so cute," says the doting mom. "She's also a bruiser, she'll just go darting off in one second and you're like, 'Wait, how'd you get all the way over there?' It's crazy. They're growing up so fast."

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Earlier this week, the singer shared a sweet back-to-school photo on Instagram of Maxwell and Ace, and Simpson tells PEOPLE she's taking schooling during the pandemic "one step at a time."

"School is about to start back up. Woo. It's like, 'Come on kids. You can do this,' " she says. "Managing all of it, you just got to take one day at a time. For me I have a lot of creative endeavors right now that I'm juggling and a lot of fun announcements that I'll be making very soon."

Simpson says her time self-isolating at home during the coronavirus outbreak has been "very enlightening experience, prayerful, [and] a journey for sure."

"I've had emotions that I've never had before and I've had to work my way through them," says Simpson. "I'm Googling lots of motivational speeches, just trying to stay focused and keep everything aligned and really just have that steadfast faith."

The star shared how engaging in mindful and prayerful talks with Maxwell helps them stay calmed and grounded.

"I put my oldest daughter to bed every night and we pray," Simpson says. "Once she falls asleep, I've realized what's calmed her is that I start just Googling positive words and I just start talking and then she's out."

"People are definitely at the place in their lives where they need to be on their knees and prayerful and aware of how much power is within you," she adds. "How resilient you are and just how much of the day can consume you — all the things — but trying to focus on the heart and soul of what really matters."