The Home Alone alum, 42, and his Dollface actress fiancée, 34, have quietly welcomed a second baby, brother Kieran Culkin confirmed while chatting with Access Hollywood at the premiere of the final season of Succession on Monday.
Asked about his expanding family at the event, Kieran explained, "The cousins live in Los Angeles and we live in New York. I have a 1-year-old and a 3-year-old, and as busy as I am, there's just no getting on a plane and going, and same with them."
"I haven't met No. 2 yet, and they haven't met No. 2 yet either," he continued, referencing his own kids with wife Jazz Charton. "We just haven't been able to figure that out."
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Macaulay and Song are already parents to son Dakota, 23 months. The private family has yet to share photos of either of their little ones.
The actress admitted that though she and Macaulay are "very hands-on" parents without a nanny, she's glad she had a strong support system as she navigated her "fourth trimester."
"My fiancé and I are very hands-on. We don't have a nanny, but my mom has been here with us since my son was born... When I was working, my mom would bring [my son] to set so I could breastfeed and see him during the day. People tell you a lot about labor and pregnancy, but not about the fourth trimester," Song told the outlet. "To my girlfriends that are pregnant I say, make sure you have help. Because your instinct is to want to do it all and you physically can't."
"I think when you and your partner have kids, especially the first few months, you don't have the patience to be polite with each other," she added. "Being communicative when you need help is so important. Instead of having set duties, we just feel each other out. I'll be putting my son down and my partner is like, 'Let me feed the animals and get dinner ready.' "