Coach,
A drill that I use really helped in the past. I call it the two ball drill. You can perform the drill with as many players as you like. You start with both balls in your possession. Girl or girls lined up across from you, ready to field a ball. You then throw either grounders or liners at the players. There only job is to catch and throw back as quick as possible. As soon as you catch the first ball, the second is on its way. Focus only on footwork and quick release. I should warn you, if there is only one girl, they get tired very fast. Multiple players, I would throw in no pattern. That way they never knew if they were getting the ball or not. Always have to be ready.