Amanda Peet, who welcomed her first child
, daughter Frances Pen, in February, is feeling the rigors of new parenthood – but says it's worth it.
"I'm very busy with my baby – I have to get back to my projectile-vomiting baby," Peet, 35, told PEOPLE at Wednesday's Oscar de la Renta boutique opening in West Hollywood benefiting the Women's Cancer Research Fund.
"In the beginning, it's very, very hard because you're so sleep-deprived. But she just started smiling three days ago, so I'm on cloud nine right now."
And Peet and her husband, screenwriter David Benioff, are already thinking about adding to their family. "I think we want more [kids]," she said. "He wants three, but I'm old, I'm an old lady. Two is enough!"
Asked who has been giving her child-rearing advice, Peet said: "My mom has been fantastic."
Benioff also has been doing his part. "Changing diapers – he's actually doing the night-feeding," Peet said. "He's letting me sleep right now. The hardest part is being so monumentally sleep-deprived."
Benioff and Peet dated for four years before they got married in September
at Peet's alma mater, Friends Seminary in New York City.