Two of the children were in critical condition, but all six were expected to survive, The Dallas Morning News reported.
"I believe we are going to be very blessed," Stacie Durham, a McKinney Fire Department spokeswoman, said. "My understanding is that most of them had their backs to it, so they didn't inhale the flames, causing injuries to the lungs."
The names of the children injured in the 1:15 p.m. Monday blast had not been released.
Durham said the children, all siblings or cousins ages 4-11, were playing near the fire pit when at least one of them sprinkled gasoline on the flames, which flared and ignited the fumes in the can.
Durham said there was an adult in the house but not outside with the children.
"It was a whoosh!" said Deb Martin, a neighbor who was the first to get to the scene. "I don't know how to describe it. And there were kids screaming. I thought maybe it was a firecracker, but they still were screaming, and I saw some of the kids running from the back yard.
"I asked if everyone was OK, and one said, 'No, fire.' I could hear the smoke alarms inside the house, and I went in to see if anyone was in there. But all I saw were kids."
Several neighbors began removing charred clothing from the children to determine the extent of their burns, the newspaper said. Martin said one girl, about 6, was burned so badly "her whole back and the back of her legs were peeling."