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Vanessa Lachey, Nick Lachey, Instagram

Instagram

Nick Lachey turned 41 and Vanessa Lachey turned 34 Sunday, but mom and dad's joint birthday at Disneyland was all about the couple's 2-year-old son Camden!

"We kind of wanted to do something special like that for him before his little sister arrives on the scene and steals his thunder," Nick told E! News Tuesday while promoting his fourth album, Soundtrack of my Life.

So is Camden excited about having a baby sister? "I think so," said Nick. "We've definitely gone over the fact that, 'Hey, baby sister is in mommy's belly,' and he'll kiss the belly, so I think he understands what baby sister means and that that's happening, but I think the truth test is when she arrives and all the attention's not just on him! We'll see how he reacts at that point!"

Nick Lachey, Soundtrack of my Life

The former 98 Degrees boy-bander said "it's anybody's guess" when the littlest Lachey will arrive. "We're probably looking at the end of the year, beginning of next year—somewhere in that time frame."

And while Nick and Vanessa's daughter could "possibly" be here by Christmas, he joked that "from a selfish standpoint," he'd "love to see it not happen in the holidays, but some things are out of our control!"

"We've decided on a name," said Nick, but he's not spilling the beans on that one just yet!

Vanessa Lachey, Nick Lachey, Instagram

Instagram

Vanessa and Nick are both "excited about having a little girl," said dad, who noted with this pregnancy there's been "a lot more clothes shopping happening than we ever did for Camden!"

Fortunately, Vanessa has "knock on wood, had a great pregnancy," said Nick. "She's been a trouper."

The Lacheys are going to have their hands full with two kids under 2 (!), but their longtime love for one another is stronger than ever. On his new album, Nick included a track that's "became a theme song of sorts for Vanessa and I" through the years—"Falling Slowly" from the movie Once. The lovebirds "actually had it performed at our wedding shower" back in 2011, said Nick, noting that even today, it's probably the song "that has the most personal meaning" to the music industry vet.

Soundtrack of My Life is available Tuesday exclusively at Walmart and online. You can also catch Nick hosting VH1's Big Morning Buzz Live every weekday at 9 am.