Mum's heartbreak over daughters killed in a car crash with their grandparents.

The mother of two young girls tragically killed in a horror truck smash has revealed her daughters, Willow and Caitlin Hamilton were off on a special outing with their grandparents, Greg and Rosemary Galatis, when they were killed.

Willow, five, and Caitlin, 10, were thrilled to be heading off on an adventure, caravaning at Willow Springs Station in the Flinders Ranges with their ‘Papou’ and ‘Nanna’, Leanne Hamilton-Kinnear told The Adelaide Advertiser

The location was a family favourite, with Willow reportedly named after the camping spot.

“It was the girls’ first time staying in the caravan and they were really looking forward to it,” she said. “It was their first trip away just with their grandparents. It was a special trip. We said goodbye to them in the morning and we took them over to Nanna and Papou’s with their bags.”

“They were on an adventure,” Hamilton-Kinnear said.

The adventure was brought to a heartbreaking halt when the family collided with a truck on the Augusta Highway.

According to The Advertiser, the accident took place just before 12.30pm on Sunday, the Subaru station wagon, towing the caravan that was to be home for the girls and their grandparents, collided with a southbound truck towing a car trailer.

The smash occured approximately 2km south of Crystal Brook, South Australia.

The truck driver was air-lifted to Royal Adelaide Hospital where he remains in a critical condition, and three passengers, two 42-year-old women and a 50-year-old man, were treated for non-threatening injuries.

Hamilton-Kinnear described her “beautiful, precious girls” as “active and colourful”.

“They were just lovely, they were vibrant, bubbly and adventurous,” she said. “They loved the outdoors, singing and dancing.”

The Advertiser report the family remembered the lives lost by adorning their front fence of their Daw Park home with colourful bows, balloons and Hawaiian leis.

“I just needed to add some colour to the front yard for the girls because they loved the garden,” Mrs Hamilton-Kinnear said.

"They would have loved it."

The family have thanked the police, who waited for them to arrive home before delivering the terrible news. Officers then took the family to pick up their eldest daughter, Hannah, who was also supposed to be with her grand-parents, but chose to visit a friend instead.

“I can't thank them enough, they were just wonderful,” Hamilton Kinnear said.

***Featured image: Screenshot via Channel 9 News.