Hello, I have been coaching my 12u DD for 6 years, and my 8u DD for 2 years now in Rec League and All Stars. Both pitch. The past 9 months or so I have been asking myself the same question regarding fitness and strength training. There are many free options out there, things you can do at your home, but it takes time to find and also takes time to put together an idea and plan that works.
I agree with one previous posted that you don't want to over do it. But strength and fitness training isn't just that. You can use this time and these exercises and motions to reinforce correct form, body movements, bat path, arm path, etc. for not just pitching, but for all aspects of the game. I have been working to put together a program for my DD and for my teams that I coach that adds fitness and strength training to practices.
For pitchers there are 2 things that I am going to focus on at the beginning of this season. One is core strength and stability. For this; Planks & side resistant weight drills are perfect. Don't do crunches, do things that forces the core to resist movement or create stability.
Here are some examples, plus more things you can so here:
http://www.saptstrength.com/blog/2015/8/13/anti-lateral-flexion-core-exercises
Also, foundational drive & stride mechanics. Work on movements from the ground to the waist that build flexibility and strength. Flexibility in the ankles, hips, and legs. And strength in the Glutes especially and anything that helps get more ground force reaction on your drive foot, and strength on the resistant ground force reaction on her stride foot. And flexibility that reinforces getting her stride leg up to close to a 90 degree angle and a good reach to maximize her stride distance in a safe and effective manner.
Lean into a wall to reinforce the angle she gets to in her stride. The title of this is for running, but I think this directly translates to the pitching motion:
http://www.fastpitchpower.com/improve-speed-running-technique/
Also do some drills and exercises that are like sprinter starts. That 'quick twitch' movement mirrors their drive and stride.
Various Crawls are great workouts you can do at home with no equipment that help strengthen and get flexibility in your core and legs:
https://redefiningstrength.com/crawling-exercises-21-fun-crawls-to-include-in-your-workouts/
This guy here is doing some good leg workouts:
https://www.wassermanstrength.com/baseball-softball-training-lunge-w-band-rotation/
In general here is a lot of free info on all sorts of strength training and fitness workouts:
http://www.fastpitchpower.com/category/free-workouts/
If you poke around these various websites I linked, you can find a wealth of info if you want to spend the time to find stuff, try them out yourself and with your DD.
And if you want to get realy deep into the weeds, check out this thread:
https://www.discussfastpitch.com/softball-pitching/18135-drive-mechanics.html
Hope that helps!