"After we were told what was wrong with her, we held each other and cried in the parking lot," her mom Cara, 39, a doctor, tells PEOPLE. "But we knew we could not let our daughter die."
Diagnosed at age 3 with Sanfilippo syndrome A – the most severe form of the genetic, debilitating disorder that prevents the body from properly breaking down sugar and causes nerve damage, brain damage, blindness and eventual death – Eliza is racing against the clock for treatment.
Cara and her husband, Glenn, found out that researchers at Nationwide Children's Hospital developed a form of gene therapy that they believe could be a cure for the syndrome, but the clinical-trial costs are an overwhelming $2.5 million.
Undaunted, the O'Neills sprang to action.
With help from filmmakers, the O'Neills created and released a video about Eliza's story and their family's efforts to get funding, connected to a Go Fund Me campaign that has so far raised more than $1 million through viral funding initiatives and other fundraising efforts.
They still need an additional $1 million by October in order for the trial to move forward.
As the effects of the disorder continue, by age 5 Eliza will most likely begin to experience irreversible brain damage, eventually losing her ability to speak, eat or walk.
"She needs the medicine now," Cara tells PEOPLE. "Before it gets very bad."
Her parents have also launched the Triple The Love Campaign, urging everyone who donates to share Eliza's story with three friends throughout June, with the goal of each donating $22.
At home, the O'Neills hold on to each moment with their daughter as she plays with her dolls and gets herself dressed in the morning, even as they hope the funding will come through early enough to help her.
"If she picks something out, we let her wear it," Cara says. "Because we are thankful that she can still dress herself."
Eliza's older brother is helping care for his sister.
"She only lets her 7-year-old brother Beckham put her to sleep at night," Glenn says. "We peek in and see that he gets into bed with her, holds her hand and talks to her until she falls asleep."