"We are so excited," former first daughter Chelsea Clinton tells PEOPLE about expecting her third child with husband Marc Mezvinsky
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Chelsea Clinton is all about her growing family.

“We are so excited,” the expectant mom told PEOPLE on Tuesday while promoting her latest project, Don't Let Them Disappear, about her third baby on the way with husband Marc Mezvinsky, due this summer. They are also parents to son Aidan, 2½, and daughter Charlotte, 4½.

The newest addition to Clinton’s author résumé is a kids’ picture book that informs readers about 12 endangered species and how they can help save them.

And while Clinton, 39, was thrilled to share the news of her pregnancy with her parents, former President Bill Clinton and former presidential candidate Hillary Clinton, the real focus was her kids, she said.

“Admittedly, the most important audience — and this is nothing against my parents or Marc’s mom — but our most important audience is now our kids,” the vice chair of the Clinton Foundation shared with PEOPLE .

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Celebrities Visit Hallmark's "Home & Family"
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EXCLUSIVE: Pregnant Chelsea Clinton Shows Her Baby Bump While All Smiling Leaving The View In New York City
Chelsea Clinton
| Credit: Splash News Online
Chelsea Clinton and daughter Charlotte
| Credit: Source: Chelsea Clinton/Twitter

“We were thrilled to share the news with my parents and my mother-in-law. They were so excited for us,” Clinton explained of her pregnancy.

Acknowledging that “every grandchild is so special,” she said it’s a “different dynamic” for her parents compared to her mother-in-law, former Congresswoman Marjorie Margolies. The next Clinton Mezvinsky will be the Clintons’ third grandchild, while he or she will be Margolies’ 23rd.

And like in her previous pregnancies, Clinton and Mezvinsky won’t find out the sex of the baby before the birth. “We didn’t find out with Charlotte or Aidan and we’re not finding out this time,” she told PEOPLE.

Chelsea Clinton Book Signing Event_BZ_04 02 19Credit: Julie Larsen Maher ©WCS
Chelsea Clinton during a reading at the Bronx Zoo
| Credit: Julie Larsen Maher ©WCS

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But that hasn’t stopped Charlotte from looking forward to the arrival of her newest baby sister or brother — and her parents were careful about how they shared the news.

“Charlotte is so excited to be a big sister again,” Clinton told PEOPLE. “And I think Aidan is mainly excited because his sister is excited. He just loves and looks up to his big sister so much.”

She continued, “Every day, [Charlotte] talks about the new baby. She wants to know how the baby is doing and she wants to feel the baby … Aidan, at 2½, doesn’t initiate that, but as soon Charlotte starts he’s like, ‘Oh, baby! Baby!’ And he’ll do a little dance, which is awesome.”

Chelsea Clinton’s new book cover
| Credit: Philomel Books

Back in 2015, shortly after giving birth for the first time, Clinton revealed to PEOPLE that becoming a mother changed her entire world.

“I didn’t know I could care more intensely about anything until I became a mom,” she explained. “Somehow I love my husband even more, I love my parents even more. I feel even greater urgency about the work that I do, particularly around women and girls, being a mom of a daughter.”

“And I think my love for Charlotte has just opened more space in my heart and my brain that I didn’t even know was there,” Clinton shared.

Don’t Let Them Disappear: 12 Endangered Species Across the Globe is on sale now at amazon.com.