If you really want to get your girls in shape, just squeezing in some conditioning at practice won't get you too far. They really need to be active 7 days a week as well as eating right and getting good rest. I would suggest for 12U that you start by implementing 3 miles per week (can be done in 2-3 days, i.e. 1 mile 3x/week) running at 80%.
I'm personally not a fan of weights at that age because most 11-12 year olds are still developing. Many will argue though that nothing detrimental will come of lifting weights at that age. BUT...you can get very nice results from doing simple exercises on a regular basis provided you are eating right as well as getting ample amounts of rest. As for how much you want to lift, you can't put a number on it for an entire group. Some will be able to lift more than others, and some will grow stronger at faster paces than others. You have to treat every individual according to what they can handle. At that age, I would recommend working out no more than 2-3 times weekly and alternating upper/lower body each workout.
Check out my blog at
http://healthyfastpitch.blogspot.com/There are several entries there that may point you in the right direction regarding nutrition. Also, our website has a very detailed conditioning program on it. If you would like to check it out I would be happy to PM you.
If your objective is truly to get your girls bigger, faster, and stronger you won't be able to do it alone. Get the parents involved. And this is key...muscles are not made in the weight room...they are made in the kitchen. Good luck!