"I'm definitely very tired," she tells PEOPLE. "I'm 42 and it's my third baby."

It’ll be a boy for Amanda Peet and David Benioff!

“I’m very excited,” the actress told PEOPLE Thursday, revealing the sex of her third child while supporting the UN Foundation and Walgreens’s Get a Shot. Give a Shot. campaign in Los Angeles.

And she’s not the only one who can’t wait to meet the newest member of the family.

“[My daughters Molly June, 4½, and Frances Pen, 7½] are very excited now,” she said. “We’ll see what happens when he actually arrives!”

As a mother of two already, the actress confessed that this pregnancy has been a bit challenging.

“I’m definitely very tired,” she said. “I’m 42 and it’s my third baby.”

Giuliana Bill Rancic Duke birthday party

Supernanny Jo Frost and Amanda Peet – Courtesy Walgreens

While Peet — who surprised fans with her happy news at the Emmys in August — admits that she “hasn’t given a lot of thought” to the nursery yet, she has been preparing her girls for their little brother’s arrival.

“We talk about it,” she says. “They talk to him and listen to him.”

For every immunization administered at Walgreens pharmacies, Walgreens will donate the value of a life-saving vaccine through the UN Foundation’s Shot@Life campaign; the year-round campaign aims to deliver six million vaccinations to children in need.

— Gabrielle Olya