Brennan Watkins’s parents thought he had a mild case of the flu when he fell ill in April. The three-year-old had a low-grade fever and was sleepy. His mother, Denise, was away on business at the time.

“His nanny called and said she thought he was getting sick,” the underwriter said. “I called the doctor and he said there was a flu going around.”

Brennan seemed to perk up on Tylenol, but the fever persisted. Then he developed a limp.

Mrs Watkins returned to Bermuda a few days later and had only just settled in at work when she received another call. Brennan was worse. His colour was bad and he was lethargic and floppy.

She and her husband rushed their son to the doctor’s office and he was sent to King Edward VII Memorial Hospital, where a blood test revealed devastating news: he had leukaemia.

“What was devastating was the unknown,” Mrs Watkins said. “At that stage we didn’t know what the treatment would be and we had all of these questions that could not be answered in the early days of diagnosis.”

The toddler was medevacked to Boston Children’s Hospital the next day. Tests there determined he had “the most common type of childhood cancer”.

B cell acute lymphoblastic leukaemia can come on suddenly in very young children. It is thought that Brennan had it for less than two months before it was diagnosed.

“It is one of the most curable types of childhood cancer,” Mrs Watkins said. “The good news is the treatment success rate is over 90 per cent. His doctor, Lewis Silverman, wrote the protocol on treating this type of cancer.”

His treatment will involve two years of chemotherapy.

“As of now, he does not have to do radiation but that might change if there are any complications that come up,” his mother said. Brennan celebrated his fourth birthday in the Massachusetts hospital. His parents threw a pizza party on May 7 for his entire floor, but it was his nanny who made the day extra special. Brennan wants to be a firefighter when he grows up. Rebecca Hall arranged for nine firefighters to pay him a surprise visit.

“One day she was walking past Engine Company 33 near our hotel in the Fenway,” Mrs Watkins said. “The garage happened to be open so she went in and told them about Brennan. They offered to come and see him on his birthday.”

The firefighters brought Lego, one of Brennan’s favourite things to play with.

“They ended up sitting there with him putting the set together,” Mrs Watkins said.

The men then visited other children on the floor.

Brennan spent 40 days in the hospital. His nurses held a special celebration for him on the day he was released.

“When children complete their first phase of treatment, the nurses hold a bubble parade,” Mrs Watkins said. “The child walks down a corridor of bubbles to Pharrell Williams’s song Happy. Brennan loved it.”

Brennan will remain in Boston as an outpatient for the next nine to 12 months. His parents are hoping he will be able to return Bermuda after that, and travel to Boston for treatment.

People in Bermuda have been very supportive, Mrs Watkins said. Family friends and Brennan’s classmates at Somersfield Academy took part in the Bermuda Cancer and Health fundraiser, Relay for Life, last weekend. Team Brennan raised $11,180.

“The money won’t help us directly,” Mrs Watkins said “But the money is going towards bringing more cancer treatment to Bermuda. I know what it’s like to have to go back and forth. If this stops another family having to go through separation during treatment, then that would be phenomenal.”

She and her underwriter husband, Michael, are trying to stay positive.

“We have met so many other families with sick children at the hospital,” she said. “Everyone’s circumstances are different and you feel for every single person. Any moment you want to feel sorry for yourself, you walk through the lobby and see a child in much worse condition than your situation.

“It is a humbling moment. Thank God this type of cancer has a good prognosis.

“Thank God he has good spirits. He’s been a real trooper. We are choosing to see the positives in this.

“Brennan will have this adventure of being in the US for about a year. Then when he’s older he’ll be able to tell people this is what he went through.

“It has been amazing to see his resiliency and bravery.”

• It’s not too late to donate to Team Brennan: visit https://runsignup.com/Race/16795/Donate/wjR0SxG3ZjuCa6nb