Mia and Daniel Spinks, aged four and six, were killed when a car driven by their mother, Sharlyne Spinks, left the road on a bend and crashed into a tree on a rural road at Peak Crossing, about 55 kilometres south-west of Brisbane CBD.

Mrs Spink's eight-year-old niece Judith Yamo was also in the car, but survived and has been kept at the Queensland Children's Hospital for observation.

Ms Spinks, aged in her 30s, remained in a critical condition at the Princess Alexandra Hospital's intensive care unit on Tuesday afternoon.

Joni Sereva who is the mother of eight-year-old Judith, took to Facebook asking friends to "stand together in prayer" for her daughter and sister, who at that stage were both still in critical conditions.

On Tuesday morning, Ms Sereva posted "Please continue to stand in prayer for Shalyne. She is still in a very critical condition at the PA [Princess Alexandra Hospital] and will need everyone's prayers".

"Judith is in a stable condition and is recovering well. We thank you all for your love and support," she wrote.

Ms Spinks was described by her other sister Samantha Sereva, as a "devoted mother and wife".

Loved ones took to social media to share their grief over the loss of two children.

One friend wrote: "My heart is just so broken to hear about this nasty accident that [has] taken away the two beautiful souls, oh I will surely miss them coming to my place our condolences to the family".

Another wrote: "RIP beautiful angels, fly high".

Margaret Kalagaus, the grandmother of the children, wrote on Facebook that she was "broken into million pieces".

"O'Lord JESUS! wwwwhhhhyyyy???," she captioned a photo of the two children on Monday evening.

The police's Ethical Standards Command has been brought in to investigate the involvement of police manning a "static traffic enforcement" position near the crash scene.

That investigation will involve reviewing body-worn camera and dashcam footage as well as the examination of the crash scene.