Get the latest from TODAY
Bailey was diagnosed in 2016, at age 8. After his chemotherapy treatments and a relapse, the Coopers, who also have a son, Riley, now 8, learned Rachel was pregnant.
"Bailey was absolutely over the moon when we told him the news, as he was going through chemotherapy again after his first relapse," Lee Cooper, Bailey's dad, told TODAY Parents.
Throughout the pregnancy, he said, Bailey loved to rest his head on his mother's stomach, talking to the baby and feeling her kick. "I was with Bailey — after he had a stem cell transplant and was in isolation — when Rachel and Riley went to have her 20-week ultrasound to find out the sex of the baby. Rachel Facetimed me and Bailey was shouting with sheer joy when they announced the baby was a girl and he was going to have a baby sister."
As Bailey continued his battle with cancer, he would suggest baby names in the car en route to treatments. When he came up with the name Millie, Lee and Rachel were sold.
Unfortunately, Bailey's condition worsened.
"When we broke the news after his second relapse that he wasn't going to survive the cancer, one of the first things he asked was if he was going to meet his baby sister before going to heaven," said Cooper.
Bailey did survive to meet baby Millie, who was born on November 30, 2017.
"Right from the day Millie was born, Bailey was smitten with her," said Cooper. "In his short time with Millie he held her, fed her, bathed her, changed her and sang to her every day until he physically couldn't do it anymore. He did everything he set out to do in meeting his sister."
Bailey died a few weeks after Millie's birth, on December 24, 2017, in a hospice facility near his Bristol, United Kingdom home. Cooper his family stayed by Bailey's bedside, keeping him comfortable, playing his favorite music and making sure he knew he was not alone in his final moments.
"On Christmas Eve, Bailey's breathing became very shallow and irregular," Cooper recalled. "Myself and Rachel sat by his bedside and we both knew it was time for him to go. We whispered in his ear, 'It's OK; we love you very much. You need to stop now. It's time for you to go.' At that moment he took his last breath and shed one tear."
Cooper says his family will work hard to remind both Millie, who turned 1 in November 2018, and their older son, Riley, of their big brother's love for them.
"We show Millie pictures of Bailey every day and every time we mention Bailey, her face lights up and she points at the canvas picture we have of him in our living room," said Cooper. "We also have a memory box with some of Bailey's things for her when she's older. Millie and Riley will be reminded of Bailey every day and we will never shy away from talking about Bailey with them."