The Moulin Rouge! star welcomed Bella and Connor in to her family with Cruise in the 1990s, when she was in her 20s.
The couple subsequently split and Kidman has since wed singer Keith Urban. She gave birth to their daughter in 2008.
Kidman, now 43, admits motherhood later on in life is very different compared to how she raised Bella and Connor, who are now teenagers - but she's convinced becoming a mum to little Sunday Rose has changed her for the better.
She tells Reuters, "I raised two kids when I was in my 20s, but that was a different thing. I was a kid and still so young mentally and emotionally - a totally different mom.
"There is something about mothering late in life. A lot of it is I want to stay alive for Sunday. There is plenty of deep fear and emotion. Sunday has healed an enormous amount in me. It's a very private thing, but she just has."