Margaret and Les Butler said Ewan Williamson, 14, was their third grandchild to have died in four years.
The Largs Academy pupil perished in 48˚C heat after collapsing on a trek through the Cape Range National Park while on a trip to see his father Gordon.
Mrs Butler, 78, from Millport on Cumbrae, said: "We lost two grandsons, Gareth and Mark, to cystic fibrosis. Gareth died in 2009 aged just 19, and Mark died in 2011, aged 18. When we heard the news that Ewan had died, we just couldn't take it in.
"I'm still dreaming about him every night. I just can't get over it. He was so excited about going to see his dad. I can't describe what it feels like to see your grandchildren getting buried before you. We've been asking: 'Why not us?' We're old and we've had our time. Not another grandson."
She said Ewan's death had impacted heavily on other family members.
"Our daughter Janet still can't talk about it and Gordon hasn't been able to speak to anyone else at all except his dad. We just don't know how one family can have so much bad luck.
"Another grandson Paul, who's 17 and the son of our son Martin, is really distraught because he thinks there is a curse on the family and that he is going to be the next one to die."
She added: "The family have been through such a tough time and we were just starting to get over things. Then this just came out of the blue."
Ewan had flown out to spend Christmas with his father Gordon, a civil engineer. They set off on the four-hour walk through Badjirrajirra Creek at about 10am. The time meant that they were out in the hottest temperatures of the day.
They went ahead with the trip despite warning signs advising of the dangers.
His father contacted emergency services and ambulance officers tended to the boy at the scene before he was taken to hospital, where he died.