@Stokesmom First off, let me say how much I appreciate that you are taking this trip on! What an incredible opportunity for these girls! I know you have lots of questions, I will do my best to answer them. Do not hesitate to reply with additional questions if you would like!
The easiest question you have is: Does REI rent gear? And the answer is Yes! The Bloomington store in Minneapolis is well equipped to outfit your band of adventurers. Feel free to give them a call or stop by for rental prices and information.
The best way to break down a trip like this is to make a list of categories that are needed and determine which are individual categories and which are group categories. The main difference being what are items that can be shared (i.e. tent, water filtration, stove, etc) and what items cannot (sleeping bag, pad, changes of clothes, food, etc.). Then I would determine how many you need of each of the shared items and assign one person to complete that mission. Generally speaking a tent is designed for a certain number of people, which should be indicated in the title (i.e. REI Co-op Half Dome 2 is made for 2 people). The baseline for that measurement is the ability to put down a 20" sleeping pad without overlapping your partners. So, a two person tent could potentially be 50" wide, providing for two 20" wide sleeping pads plus some extra room on each side. All of that is to say, with potentially 12 girls and 4 adults I would plan on taking, at minimum, four 4-person tents or six 3-person tents (or some combination of the two. It will depend on if you would like to have an adult in each tent or if you trust members of your group to sleep without an adult :). The 4 person tents will be significantly bulkier, however you can spread the pieces of the tent across multiple people to mitigate that issue (tent body, fly, poles, footprint, etc). I would recommend planning some extra room in case the weather is bad and you need to spend significant time inside the tents or bring gear inside.
I would recommend taking a backpacking checklist and giving it to each attendee to have them let you know what they have available for the trip. The one I linked is a very robust list leaving nothing out. Obviously everyone will not need a compass or a stove. So you may want to adapt that list into group gear and individual gear like I talked about earlier. Once you have them all back you can go into the Bloomington REI and speak with someone in rentals about how to fill out the remainder of your gear needs. I am sure if you gave them a call ahead of time they would be more than willing to carve out some extra time to help you plan and strategize your trip! You can even schedule an outfitting appointment if that works for you.
In terms of sleeping bags and temperature rating I would think that you would be fine with sleeping bags in the 30 degree range would be fine. Almost any trip in October would have potential for freezing temps and you'd want to be ready. As a general rule you don't want to make a child carry more than 10-20% of their weight in a backpack. That would mean that a 30-45 liter sized backpack would more than likely be quite adequate for most of your girls.
The most important thing is to take your time, ask lots of questions, and remember that this is an awesome opportunity for both you and these girls. Thank you so much for engaging young people in the outdoors!
I hope this helps, please let me know if I can answer any more questions or provide more detail. Thank you!