The question was poised carefully last January to Charlotte "Charlie" Godish, then 4: "Your brother is sick and to get better, he needs your blood cells," Brian and Jennifer Godish of Elgin, Illinois, told their shy and softspoken daughter. "We're wondering, how would you feel about helping him?"

After pondering the question for a moment, Charlie suddenly smiled and said, "Okay, I'll do it. I want to help Bradley. Let me know when you need me.'"

Now six months have passed since Bradley Godish's stem cell transplant in February to eradicate an aggressive form of leukemia. He and Charlie, his fraternal twin, have just started kindergarten, are back to their usual antics at home and are showing no ill effects from the surgery.

"They've both bounced back nicely - our hope is that the further out we get from the transplant, the better the chances are that it will never come back and he'll have a full recovery," says the twins' transplant coordinator, Jennifer Schneiderman, of the Ann and Robert Lurie Children's Hospital of Chicago.

If Charlie and Bradley were identical twins, the transplant wouldn't have been recommended, Schneiderman tells PEOPLE, "because identical twins have a higher risk of carrying the same type of leukemia. But as fraternal twins, Charlie had the same chance of matching Bradley as any sibling donor. In this case, she was the very best match we could find. She's a very brave little girl who didn't complain or cry once."

Charlie is a champion and an inspiration, her father, Brian 36, an account manager for a video and display company, tells PEOPLE. "As a parent, you want to be a hero to your child, but our kids have been heroes to us. And this whole ordeal has brought the two of them even closer."

Although they are twins, Charlie and Bradley (the oldest by one minute), couldn't be more different, says Jennifer, 36, a part-time community college counselor, who also cares for another son, 14-month-old Camden.

Bradley is blonde and left-handed while Charlie is dark-haired and right-handed, "and they each have very different personalities," she tells PEOPLE. "Charlotte has always been the bashful one, while Bradley has always been outspoken and protective of her. But when Bradley got sick, they each seemed to switch roles."

Symptoms that Bradley had something physically wrong weren't apparent at first when he began complaining that his feet hurt just before Halloween last year.

"I'd just had the baby and Bradley kept saying that he wanted to be carried," she says. "I thought he was just a little jealous and wanted some extra attention."

When his pleas to be carried by his parents continued, the Godish's took him to see a doctor and were told that he likely had a virus. But then, a few nights later, Jennifer was reading the Jodi Picoult novel, Leaving Time, and came across a passage that mentioned a connection between growing pains and leukemia.

"It was a like a light bulb went off," she says. "I couldn't get it out of my head. I thought, 'Could this be?' "

Another doctor's appointment and extensive lab work in November confirmed Jennifer's fears: Bradley had leukemia. And there was more bad news.

He had an aggressive form more common in older adults called acute myeloid leukemia that left him at high risk for a relapse after treatment. The most promising option to treat him was four rounds of chemotherapy followed by a bone marrow transplant.

"We wanted Charlie to feel like she was part of the decision-making process and wanted to prep her mentally," says Jennifer. "So we explained that Bradley's blood was sick and hers was healthy, and she said, 'Let's do it.' She's always been a giving kid."

The transplant on February 17 was trouble-free, with Charlie experiencing only minor pain for a few days afterward.

"She had a big bandage on her lower back and she didn't want to take it off because it was her badge of honor," says Brian. "She wanted to show everyone that she'd helped Bradley."

Bradley now has to be evaluated every two weeks to make sure that he isn't relapsing, says Jennifer, "but we're optimistic that all will be fine. We want him and Charlie to remember this time in their lives and how they were there for each other. The bond and the love they have as twins is now even stronger."