I love this idea, as I have been personally building both recreational and commercial (making money by filming for real estate) and I know it is a great way to learn and is also very fun. I would recommend giving
www.hobbyking.com a look as they should have everything you need to get started, with very inexpensive parts that are quite good. I would highly recommend their KK2.1.5 board, which is very stable and only $30. Be sure to have the kids learn on a flight simulator first, as these things are not the easiest to fly without gps equipment which costs an awful amount of money. Also, there is a vast database of knowledge on forums like rcgroups.com and also on flitetest.com's articles, which have far more information than I could ever suggest on a CD post. Good luck, and above all, Happy Flying!