Since you have kids... the Black Hills is really great for them.
Might I suggest that you let the kids do a little Googling of Glacier, Yellowstone and Black Hills. Let them help plan the vacation. They'll be more interested in it and get more out of it if they help plan it. As for how much time, I'd hate to say. For lots of vacationers it's the destination point. A week there wouldn't be unusual at all.
I'm sure you can order (free) vacation guides for Wyoming, Montana and South Dakota that will come packed with lots of popular tourist attractions including Yellowstone, Glacier and the Black Hills.