Stand behind them and grab a hand full of jersey. Have another coach put the bunt down on the play. Move and drag the player with you to the base she needs to be covering. After being drug around like a rag doll a few times they will figure out just how hard you want them to move to cover a base. There are always a a bunch of laughs and giggles the first few times. After that they all figure out what needs to be. done.
Never grab a kid by the jersey. I hope that was a joke.
Try putting a stopwatch to the girls and emphasize they need to be faster. Do the drills without a ball in play so they only concentrate on getting to the base as quickly as possible.
When emphasizing this be careful that the Players do not become too concerned about where they need to get to instead of playing the ball 1st. 1st base is an easy example. I have seen a lot of 1st basemen at this age whose 1st step is towards the base then need to start moving right if the ball is hit to that side of them. RF 1st step is towards backing up 1st, then recovers to go after the ball. Hopefully I am explaining this right.