The mum, said to be in her 30s, was walking to Handsworth Park in Birmingham with sons, four and 14, and daughters, seven and 12, when they were hit.
The four-year-old boy also suffered serious head injuries and was resuscitated by medics at the scene after he stopped breathing for a short time.
He added: "A woman in her 30s suffered serious head and pelvic injuries in the crash. She received emergency treatment at the scene from medics and was transferred to Queen Elizabeth Hospital Major Trauma Unit for further emergency treatment.
"A girl, believed to be 12 years old, suffered serious head injuries and also back injuries. She also received emergency treatment at the scene by medics to stabilise her condition.
"A boy, believed to be 14 years old, also suffered a head injury.
"Unfortunately the patients' injuries are considered life threatening."
They added: “West Midlands Ambulance Service would like to pay tribute to its staff for their professionalism and the excellent treatment they provided at what was an extremely difficult and distressing incident.
“The Trust would also like to thank an off duty nurse and Police Officers who assisted with treatment at the scene.”
A man has since been arrested over the incident.
A West Midlands Police spokeswoman said: "A 35-year-old local man has been arrested on suspicion of dangerous driving."