A 4-year-old boy was killed and a young girl was critically injured when a box truck rear-ended a car at a stoplight in Davenport on Saturday morning, authorities said.

Around 9:15 a.m, a white Toyota was stopped at traffic signal on U.S. 27 at Polo Park Boulevard when a Freightliner truck approached from the behind. The truck driver, Ednaldo Marques, 18, of Winter Garden, was not able to stop, even after using emergency brakes. He ran into the Toyota, the Polk County Sheriff's Office said.

Investigators say the back of the Toyota was pushed into the passenger compartment, where the two young children were seated.

Witnesses rushed to help the trapped family, including Kevin Simmons.

"So I pulled over on the intersection and I took my jack out trying to break the passenger window, trying to get to the driver, not realizing there were two little kids in the back," said Simmons.

Four-year-old Nicholas Caroll died at the scene, and 2-year-old MacKenzie Carroll was transported to Arnold Palmer Regional Medical Center in critical condition.

"I have a daughter and a son and my beautiful wife Camille, and when I saw the first responders take him out it made me think of my son," Simmons said.

The children's father, Robert Carroll, 41, of Clermont, was transported to Orlando Regional Medical Center in serious condition. Marques also was taken to a hospital. He hurt his hand trying to break a window in the car to help the Carroll family.

The impact of the crash also pushed the Toyota into the rear of a 2006 Dodge Ram truck, driven by 52-year-old John Dean, also of Clermont. Neither he nor his wife were hurt.

South U.S. 27 was closed for five hours because of the crash.

This was Polk County's second fatal rear-end crash in eight days. On May 29, Christopher Ivey, 38, of Lake Wales and his 4-year-old son were killed when they were hit from behind by an alleged impaired driver.