Excellent question. Your heart is in the right place.
At that age, I suggest:
(A) Individual defensive skill work (20 min)
(B) Team defensive skills (15 min)
(C) Batting (30 min)
(D) Game (20-30)
Individual skill work
One group goes with one coach for popups. The other goes for grounders. Both are done the same way--you line the girls up, you hit a grounder/pop-up, then the next girls gets in line. (IMPORTANT: hit, do not throw, grounders or pop-ups). You want to do hit these as quickly as you can. Keep the line moving. After 10 minutes, the groups switch.
As long as you keep the kids moving, they'll be fine.
Team defensive skill
I hate this kind of work, but at your age group, you've got to spend some time with the girls "in position". you put them in position, and then you hit them balls, and tell them where to throw the ball.
Do it for 15 minutes, every practice. When the girls start spacing out (and they will), stop and have them run a lap around the bases.
The goal is for each kid to swing the bat 100 times. So, you need all kinds of stations set up--batting tees, soft toss, live pitching, wiffle balls, dry swings, etc.
Always end with a game. My favorite is "shotgun". You divide your team into three groups. one group bats, one group plays infield, one group plays outfield.
the coach pitches. The batter gets 1 or 2 swings. A point is scored for each base the batter reaches successfully. The batter never stays on the bases, but comes in. After 3 outs or after that group has batted around twice, rotate the groups.
Keep it moving, and "play" with the kids. Tease them, joke with them, give one team an unfair advantage, make the best kid hit left handed, whatever. Just have fun.
Be very careful in picking your assistant coaches. There are three types:
(A) Mr/Mrs. Jock--"I played one year of HS baseball. I watched the Cubs in spring training. Therefore, I know all there is to know about softball."
(B) Mr/Mrs Wonderful--"These girls are so sweet. They are so nice. Oh, gosh, I hope they don't hurt themselves. I don't want them to break a fingernail. Oh, and here are some pretty ribbons for their hair."
(C) Mr/Mrs "Let get the job done"--"Life is short, kids are fun. Where do we start?"