So, of course, both were thrilled to see Jennifer Garner suddenly walk into Henry's hospital room at the Aflac Cancer Center of Children's Healthcare of Atlanta last Sunday, where the brave youngster has undergone 17 rounds of aggressive chemotherapy in his battle against a cancer known as osteosarcoma.
"I think she was flattered that he recognized her," Carrie told PEOPLE in describing the visit, which she said was sweet and uplifting.
"She visited with us for a while, and it's hard to explain, it just felt so natural," Carrie says. "She was so down to earth, so genuine."
Garner, 43, who has been going through a painful split from husband Ben Affleck, has been filming the movie Miracles from Heaven in Atlanta. On Sunday, she and costar Kylie Rogers, 11, decided to make an impromptu visit to patients at the Scottish Rite hospital.
"Someone said they saw her crying at the nurses' station after seeing all of the sick children with cancer. I think it really got to her, especially as a mother," Carrie says. "She is a very sweet person with a very big heart."
Garner and Rogers took photos with Henry and his twin sister, but Carrie declined to get a photo with the actress herself, explaining that she'd been at the hospital all night and hadn't had a chance to shower or put on makeup.
"She said, 'I have three kids, believe me, I understand,' " says Carrie.
Henry was diagnosed last year after a pain in his knee turned out to be osteosarcoma, a type of bone cancer. In March, he underwent a rotationplasty, which involved removing his knee and rotating and reattaching his ankle where his knee had been – so he'll have better control over a prosthetic leg when he is fitted for one.
He will have his 18th and final round of chemo this weekend.
Henry's family have set up a GiveForward page to raise money for his future health expenses. And they hope his experience will help raise awareness about childhood cancer.
"There needs to be a lot more awareness and research done, because it is so different than adults with cancer," says Carrie. "And who are more important than our children?"