A six-year-old boy is in a critical condition after repeatedly pinning himself against a wall while playing with a full-sized remote-controlled car, emergency services say.

He suffered multiple leg fractures, pelvic and head injuries at his family's semi-rural property at Grose Vale, in Sydney's north-west, just before 11am today.

"It is thought from the information that people gave to doctors at the scene that somehow the boy hit the button and the car has gone forward and pinned him against the wall," NRMA CareFlight spokesman Ian Badham said.

"That happened four times - the car pinned him four times. That's why both legs have been badly broken."

Emergency services were called, but the boy's mother put him in the car and started to drive to hospital, he said.