Wow Christie, good for you for thinking down this track!
Linda and others will have much better advice than I can give, but I do think you should do something like Linda has done and set portraits up in categories maybe with discounts or incentives for life settings.
You are gonna get those kids that just won't sit still even for 5 minutes for photography, let alone setting up and painting them from life. Others will do better, but I think you will have to learn to work fast! Maybe with a combination of life and photography on the younger ones.
The thing I have found, is that even one sitting from life will improve the painting - everything from value in highlights and shadow depth, to skin tone and temperature can be suddenly seen as it actually is - not how your camera has depicted it.
Please share how you do with this, okay?