The couple has been romantically linked since 2017 and welcomed their first child, Dakota, in April 2021. Though Culkin and Song tend to be private about their relationship, they gush about each other from time to time. In honor of the Home Alone star's 40th birthday, Song wrote him a very sweet message.
"Happy 40th birthday to this magical being," Song captioned an Instagram post in 2020. "I could sit here and write endlessly about how wonderful, kind, loving, genuine, loyal, honest, brilliant and hilarious you are, and how grateful I am that I get to share and do this life thing with you."
She continued, "But firstly, that would take me an eternity and secondly, you won't even see this because you don't ever use Instagram. Hahaha. My unicorn that I never thought could exist, I am luckiest person in the world because I am loved by you."
See the Instagram post below.
View this post on Instagram