The explosion severed the left foot of Kristina Perez, 32, and left fourth-degree burns on the left leg of Lorena Perez, 34, WGN said.
"One of the firecrackers, instead of going up, went to the ground and hit Kristina," her father, Eleuterio Carmona, told WGN.
The women said they were watching a fireworks display at the park when the explosion happened. Both were in serious condition at a hospital Friday, WGN said.
The display was not an official, regulated show, the station reported.
Illegal fireworks destroy 14 boats
Seattle authorities said illegal fireworks sparked a massive fire that caused $1.5 million in damage.
A firework landed on a boat cover at a boat storage facility on the north end of Lake Union, Seattle Fire Department spokesman Kyle Moore said. He said the dry storage facility stacks boats four stories high.
Within seven minutes, the fire had destroyed 14 boats.
Seattle also had three roof fires started by fireworks, according to Moore.