So, like many parents, Kelly and Alan Montgomery are keen to give their little boy the "best party ever" to celebrate him turning one in a couple of weeks.

But tragically for the couple from Windang on NSW's south coast, they've just found out that this may be the only birthday Ryder ever has.

A few weeks ago, they took Ryder for a check-up for a cold, and - almost as an afterthought - asked the doctor to look at his face, as they had noticed one eye didn't close properly when he cried.

IN OTHER NEWS:

Concerned, the doctor sent them to Wollongong Hospital, where Ryder was sent for an MRI.

"It came back that he had quite a large tumour on his brain stem, so we got transferred to Randwick Children's Hospital and they did surgery," Ms Montgomery said.

"It didn't go as well as planned, because they could only take about 20 per cent of the tumour because of where it's located near a critical part of the brain and entangled in a lot of nerves."

"They sent the tumour overseas to find out what it was, and it's only just come back that it's a rare tumour, but more common in kids under five."

Now, Ms Montgomery said the family have been told Ryder has a 10 per cent chance his treatment will work, which is further complicated by his young age.

"They've just got to wait and see, so we'll see how it goes, and if the tumour grows or doesn't respond, then they'll have to stop," she said.

"It's hard to know what the right thing to do is, we're making this big decision for him - but we've got to try."

After being in hospital for more than a month, Ryder is due to head home later this week before he begins an intensive 12 month plan involving chemotherapy and radiotherapy. He'll be back in hospital on his first birthday in mid May.

"I thought, if we can get him a birthday party while he's fairly well and not sick with the chemo, that would be good," Ms Montgomery said.

"We weren't really planning to have a big birthday for him, but now I don't care about anything else, I just want to give him the best party ever.

"I think he will love it - he loves his family, and I want photos and for him to have the best day."

Last week, the family's friends reached out to local businesses to ask for support to make Ryder's party special, and said they have been overwhelmed by people offering food, balloons, invitations and money for the big day, which will be held at Lake Illawarra Hotel on Sunday.

"They don't even know Ryder and they've just volunteered their time and money for him, it's really overwhelming," Ms Montgomery said.

Unbelievably, this is not the first time Kelly and Alan have watched one of their kids go through a serious illness.

When their now three-year-old daughter Charlie was six months old, she was rushed to Westmead with a heart condition.

"Two days later she had surgery, and 10 days after that she was out of hospital," Ms Montgomery.

"They picked it up before something bad happened, and she is thriving now, she just needs a yearly check. But that was a scary time too."

"It's so hard to watch your kids go through this, but you've got to be strong and you've just got to do it. I think I've been living in a fog, it doesn't seem real."

Just before Ryder got sick, he had started day care and Ms Montgomery was about to return to work. But now, she says her whole family's life has been put on hold as they wait to find out whether Ryder's tumour will respond to treatment.

"The weekend we took him to hospital was when I was meant to go back to work, but that didn't quite happen," she said

With Mr Montgomery also not working as the family manages Ryder's treatment and cares for three-year-old Charlie, their friends have started a GoFundMe account to help them focus on their children over the coming months.