"They can't believe how much their worlds have turned upside down – and are already thinking 'Why did we leave this so long?'," the source continued.
While the new member of their family arrived only a couple of weeks ago, the insider revealed why they are keen to add to their brood so soon.
"They are thinking - let's give her a brother or sister, let's make a bigger family," the source told The Sun.
"[Benji is] a twin so would love for his daughter to have a sibling close in age."
"Happy New Year from the Maddens!" both stars shared on the social media platform.
"We are so happy, blessed and grateful to begin this new decade by announcing the birth of our daughter, Raddix Madden. She has instantly captured our hearts and completed our family."
It was later revealed that the couple used a surrogate to finally achieve their dream of having a child.
According to Us Weekly, the Hollywood couple welcomed their daughter, Raddix into the world via a surrogate mother on December 30.
Sources close to the celebrity couple reportedly told the publication that Cameron and Benji enlisted the help of a surrogate after years of trying to fall pregnant naturally.
"They went through so much to get to this point… Cameron feels like this baby is truly a miracle," the source said.