When Caitlin's daughter got sore legs on their holiday, she could've never imagined the true cause.

Caitlin's family trip to Hamilton Island was meant to be a carefree tropical escape. Instead, it became the beginning of a nightmare no parent should face.

When her usually energetic 10-year-old daughter Sienna started complaining about sore legs and stomach pain, maternal instinct told Caitlin something was seriously wrong.

A GP visit initially dismissed it as a tummy bug. But Caitlin couldn't shake the feeling that something more sinister was happening.

In the weeks that followed, the signs multiplied. Sienna began limping during soccer. She wasn't herself at her school Book Week parade.

"I look back on photos from that day and I can see it in her face. I knew something was going on," Caitlin told Mamamia.

Watch: Lea's cancer story. Post continues below.

Then came the rash, spreading rapidly across Sienna's body. By morning, Caitlin knew they couldn't wait any longer and rushed her daughter to hospital.

"As a mother, all of these different thoughts had come to my mind … you doubt yourself because she'd have good days and then some days a bit tired," she said.

But she never imagined the words that would come next. Sienna had leukaemia.

"When I got the news, it was like an out-of-body experience. My daughter was asking if she's going to die. I think I was just crying constantly and saying sorry," Caitlin said.

"My poor husband got the news in a public space, stuck on a train. I wish I'd held off on calling him but I never knew it would be that.

"Never in my nightmare did I think those words would be said."

Siblings Mace, 7, Scarlet, 13, and Sienna, 11, at Hamilton Island. Image: Supplied.

A whirlwind of treatment begins.

The diagnosis, B-cell acute lymphoblastic leukaemia, sent the family into a whirlwind of hospital visits and treatments. Though considered the most curable childhood cancer, the treatment journey is brutal.

Within days, Sienna was undergoing chemotherapy at the Children's Hospital at Westmead. The side effects were devastating. Sienna quickly lost her hair and grew more frail.

"It's horrible for any child but especially hard to watch for a girl. It puts the stamp on your child being sick," Caitlin said.

Over 10 gruelling months, Sienna endured intense chemotherapy that triggered two strokes and countless hospital stays.

"The poor kid has experienced listening to kids have seizures, we've lost friends," Caitlin said.

But amidst the darkness came unexpected light: the fierce community of cancer families who understood their journey like no one else could.

"As a parent, that's so important to relate to people that are going through the same thing because you've just got to get on with it for your child but there's a lot of trauma that comes with that," Caitlin said.

Sienna in hospital with her family. Image: Supplied.

Since finishing her treatment in June, Sienna's days now involve oral chemotherapy until August. But her family lives in a state of cautious hope.

It's a "wait and watch situation" — something that's exciting but daunting for her parents.

"In this journey you lean towards the next hope," Caitlin said.

"First, you don't want it to be cancer, then you hope it's the 'best' cancer, then it's how is she going to respond, then how's she going to do when she goes onto oral.

"Now the hope is, 'Okay we've got to get through the next five months' and then the hope is there's not one cancer cell left in her blood."

Sienna in treatment. Image: Supplied. 

'You don't have a choice.'

Throughout Sienna's cancer journey, well-meaning friends and loved ones commented on her strength, telling Caitlin: "I don't know how you do it."

But her reality is heartbreakingly simple: "You don't have a choice."

Caitlin's family faced the additional challenge of balancing Sienna's treatment with the needs of their other children. Rather than uprooting everyone's lives from the Central Coast to Sydney, Caitlin and her husband Scott took turns at the hospital.

"You become a tag team but you've got to stay strong," she said.

It's easy to let your relationship fall aside, because it becomes the last thing that's a priority for you, Caitlin admitted.

"But you have to dig deep and remind yourself that this isn't normal and build your relationship off it not being normal.

"A lot of marriages don't make it out of these times. If you do, it bonds you. It makes you really strong."

The family takes things day by day, moment by moment, because they know just how quickly things can change.

"It's not normal, it's a new normal. There's hope one day it will be, but we're constantly living on edge," Caitlin said.

Scott and Caitlin with their three kids. Image: Supplied.

The aftermath that no one talks about.

It's what happens after treatment that Caitlin now wants people to understand. The trauma that lingers once you leave the hospital, and suddenly, you're on your own.

"I was diagnosed with PTSD in August last year. After we got home, it all flooded through and hit me like a tonne of bricks — the reality of what we've just gone through," Caitlin said.

"The more I talk to other parents of sick kids, they're experiencing the same thing.

"There's not much support when you're told you can go home after experiencing all that scary stuff."

This realisation has inspired Caitlin to completely change her career path after 12 years. She's now training in trauma family therapy, determined to support other families facing the aftermath of childhood illness.

"I want to give back to the world," she said.

Caitlin and Sienna are sharing their story in support of the Leukaemia Foundation's World's Greatest Shave, supporting the 150,000 Australians facing blood cancer. 

The Leukaemia Foundation urges the Australian community to sign up to shave, cut, or colour their hair for the World's Greatest Shave and help join the fight against blood cancer. Register at worldsgreatestshave.com or call 1800 500 088. 

Feature image: Supplied.