One Lexington child is still in the hospital in serious condition after he, his brother and sister were involved in a car accident early Monday evening.

One Lexington child is still in the hospital in serious condition after he, his brother and sister were involved in a car accident early Monday evening.

At 5:25 p.m. on Monday, the 3-year-old boy was riding in a plastic wagon with his older brother, 6, and his younger sister, 14 months, on Winter Street, according to police.

“It appears the children were riding in a wagon that rolled down a hill and into traffic on Bedford Street,” said Lt. Joseph O’Leary.

The wagon was then struck by a minivan, injuring the two youngest children.

The younger boy was transported to Hanscom by ambulance and then was flown to Boston Medical Center. The girl was transported to Boston Medical Center in an ambulance and was released last night.

Police are still investigating the cause of the crash and if the children were being supervised by any adults, said Lt. Manuel Ferro. As of now, police say they are unlikely to file any charges against the driver or anyone else involved.

Reporter Meghan B. Kelly can be reached at mkelly@cnc.com.