KidsAndCars said late Monday that it had learned of two additional children -- one in Florida and another in Missouri -- who had died after being found in hot cars this weekend.
"We simply cannot accept these deaths as tragedies and move on," the organization said in a news release.
In Missouri, the McDonald County Sheriff's Office said that a 2-year-old boy, identified by his family as Raiden Wells, had been found unconscious around 3 p.m. Saturday inside a vehicle parked in the yard of a home in Rocky Comfort.
The boy and other children had been playing in the yard as the father checked on them repeatedly, authorities said. When the father did not see the boy with the others, he went looking and found him in the back floorboard of the four-door vehicle, authorities said.
The little boy was taken to a hospital in Joplin, Missouri, where he was pronounced dead.
Rocky Comfort was under an excessive heat warning that weekend, with temperatures in the mid- to upper-90s as well as high humidity, according to the National Weather Service.
Authorities said its Children's Division would be investigating.
In Dallas, police were still investigating the death of a 2-year-old boy who had been left in a hot car as his family attended church Sunday. The little boy's death made him the fifth child to die in a hot car in Texas this year.
National Heatstroke Awareness Day is Sunday, according to the National Child Passenger Safety Board. The board offered several safety tips, including never leaving a child in a car; calling 911 if you see a child unattended in a car; and always locking your car and teaching your children not to play in vehicles.
ABC News' Becky Perlow and Casey Decker contributed to this story.