There's a strong connection between Hamm and Arroyo, who, thanks to the National Marrow Donor Program's registry, can run and play soccer with the other kids.
Hamm and her husband, Dodgers first baseman Nomar Garciaparra, were on hand Sunday afternoon at his old high school to kick off a program to help patients and their families in their fight against cancer.
The two call their program "9 to 5" because they consider giving back to the community a full-time effort.
While youngsters scurried about on the St. John Bosco High School field, adults filled out forms and had swabs taken to register with the marrow donor program.
"Being a donor can be a life-altering experience. It's an opportunity to help change the world," Hamm said.
Garciaparra said, "Just registering can save a person's life."
Hamm, who began a foundation to support research on diseases of the bone marrow, had a tragic personal experience: her brother Garrett died at 28 of aplastic anemia, a rare blood disorder. He did receive a bone marrow transplant, but other complications caused his death in 1997.
She and Garciaparra are teaming with Children's Hospital Los Angeles and plan to put together a fundraising soccer match next year that will feature stars from sports and the entertainment industry.
Hamm put her arm around Anthony, noted that he had a marrow transplant after a match was found in the registry, and asked him how he was feeling.
"Good," he shot back, grinning.
Anthony's parents, Ron and Juanita Arroyo, beamed afterward as they watched him running around the field.
"We feel very lucky," Juanita Arroyo said. "They were able to find a perfect (marrow) match for him."
Dr. Neena Kapoor, Anthony's doctor, watched him chasing around and said, "It's always amazing. It's the best reward we can get."
Hamm's due date is around mid-April, and she said with a laugh, "I can feel some kicks. I can't decide whether that's like me or like Nomar's fidgeting when he's playing baseball."
With an obvious soccer mom and baseball dad, what sport will their children favor? Said Garciaparra: "That will be entirely up to them."