A toddler is fighting for life after she and her family were hit by a metal starting gate at a harness race track north of Brisbane.

The family, believed to be a harness racing family from the Lockyer Valley, were spectators at the Redcliffe Paceway when the gate on a pace vehicle failed to retract and struck them on Sunday night.

A two-year-old girl with critical facial and abdominal injuries was transported by road to the Queensland Children's Hospital.

The girl's one-year-old brother was also injured, and spent the night in hospital with non-life threatening injuries.

Her mother and father were also injured.

A workplace health and safety investigating is now underway to determine what went wrong.

Police are assisting with the investigation.

The ambulance service said paramedics faced a chaotic scene when they arrived, but worked quickly to help the family.

The Redcliffe Harness Racing Club has used social media to praise those who assisted.

"Our thoughts and love with those injured in the mobile incident," the club posted on Facebook.

"Thanks to everyone who helped so quickly."

Racing Queensland has cancelled Wednesday's meeting at the Redcliffe Paceway, and has offered a counselling service for anyone affected by the incident.