It sounds like a wonderful trip!
I second the thoughts about Lamar Valley - it's absolutely beautiful and a good chance to see wildlife, but it's most worthwhile if you can get there very early in the morning. You will likely have plenty of other chances to see wildlife in the park.
I have one other recommendation. Even after researching the heck out of everything about Yellowstone, I was still kind of overwhelmed with getting around in the park. We ended up buying an app called the Gypsy Guide (it was a few dollars, the only app I've bought other than Paprika, lol). It was incredibly helpful. It uses GPT to track where you are in the park and narrates for you - tells you about the upcoming stops, history and other facts about the park. I found it really helpful to refresh my memory about each area of the park, plus the kids were open to listening to the narrator, whereas they would NOT have listened if I was reading it aloud.
-Christine
DD, 9/07
DS, 5/09
and a very tolerant pup