The game of softball is played entirely on the balls of your feet. To generate speed you must realize the dynamics of running. If you are running with yours heels hitting the ground first, you are actually pulling the ground towards you. This is not generating any speed. When you run on the balls of your feet and lean forward, you are pushing the ground away from you and this generates speed.
Have the girls practice running to first base. The first few steps should be short, choppy steps and they should still be in a bit of a crouch and leaning forward. After about ten feet they should begin to straighten up just a bit and continue leaning forward. When you reach half way to first base, begin to pump your arms as fast as you can. This will make your legs move faster and don't stop until you go past first base.