A source tells PEOPLE the 34-year-old actress introduced the rapper, 30, to her three kids: sons Noah Shannon, 7, Bodhi Ransom, 6, and Journey River, 3. She shares the three with estranged husband Brian Austin Green.
"He recently met her kids for the first time. Megan thought it was a natural step, since they are getting more serious," the source says. "They are in a committed relationship and it's not just a fling. They are planning a future together."
The source adds that the two "spend as much time together as they can" and are "doing great."
Fox and Kelly went public with their relationship this summer after photos of them spending time together started circulating in May, prompting Green to confirm that he had separated with the Transformers late last year after 10 years of marriage.
(from left the right) Machine Gun Kelly, Megan Fox, Briand Austin Green
Last month, Kelly spoke about their blossoming romance on The Howard Stern Show, saying that he fell in love "for the first time" with Fox.
"That was my first experience with being open to love and stuff like that. I definitely wasn't set up to believe that that's something that could ever exist," the "Bloody Valentine" rapper said.
When asked by Stern if it was love at first sight for the two of them, Kelly said, "Yeah, for sure."
Megan Fox/ Instagram Machine Gun Kelly and Megan Fox
As for Green, he recently shared on Instagram Live that although he and Fox are on different paths at the moment, it’s impossible to tell what the future holds.
“I never say never,” Green said when asked about the possibility of one day getting back together. “I kinda feel like people are on paths in life and sometimes your paths are together and you travel that path together and you see eye to eye and then sometimes paths do different things.”
“We had an amazing 15-year relationship. We have three beautiful kids. We shared a lot together and we really went through a lot together,” he continued. “Right now the paths are different and she is on her path doing what she feels she needs to be doing to be happy and I’m on my path doing what I feel I need to do to be happy and it’s not for a lack of love for the kids or lack of being responsible.”