"There is no doubt that she is starting to show symptoms," her dad Glenn, 41, tells PEOPLE. "It's getting hard for her to focus, and at times it's difficult for her to speak."
Since Eliza's diagnosis in July 2013 with Sanfilippo syndrome A – the most severe form of the rare genetic disorder – Eliza, along with her family, are racing to meet their deadline of raising $2.5 million for a clinical trial that could possibly save not only her life, but also the lives of other children suffering from the disorder.
When PEOPLE first wrote about the O'Neills, they needed to raise another $1 million by the end of the year.
They now need an additional $650,000 or else the trial, which would take place at Nationwide Children's Hospital in Ohio, may not proceed as planned.
"Our life now is about saving Eliza," says Glenn, whose wife Cara, 39, has had to leave her job to stay at home. "We can't just sit back and wait for her to die."
By the age of 5, their spunky and playful daughter could have irreversible neurological damage that will then lead to death.
But the South Carolina couple are doing everything in their power to give their daughter a shot at not just surviving, but thriving.
The O'Neill Family
Similar to the ALS Ice Bucket Challenge, the O'Neills are challenging people to #Sing2Lines of any song on video and then tag five friends on Facebook to do the same.
So far, thousands of people have completed the challenge, which has raised $75,000.
Actress Andie MacDowell has taken it, and country music singer Wynonna Judd has Tweeted about her interest in it.
"We need to get more people to do this," Glenn says. "Or it will be too late."
At home, the O'Neills stay hopeful but are taking extreme measures to keep Eliza safe.
They have only left their home a handful of times in the last four months because they're afraid Eliza will get sick and not be a great candidate for the trial.
"Eliza is going to get the medicine at Christmastime," her brother Beckham, 7, tells his parents. "It will be her Christmas present."