Hi mmrunsdisney--I do think you could arrange most of this itinerary on your own. I think you would miss out on the following:
1.) The private lunch at the Jackson Playhouse. This is alot of fun as the waiters/waitresses are actually actors starring in the what is running at the theatre. So, there is alot of entertainment during the meal.
2.) We had a Park Ranger for one of the days in Yellowstone. We went to 3 different sections of Yellowstone that day and she was a wealth of information. I'm not sure if they do Ranger led walks for the general public at Yellowstone (didn't see any). If they do, it may be a similar experience, if they don't the alternative is reading the signs in front of the geothermal feature.
3.) My understanding is that it is very difficult to get reservations in Yellowstone. Obviously, if you plan ahead and book a year in advance, this would not be an issue.
4.) I was talking to one of the wranglers at Brooks Lake Lodge and he said reservations are pretty hard to come by there also. The Lodge and property are exclusive to Disney when there is an
ABD there. He also said that they have a core of guests that return every year, so they tend to fill up pretty quickly. My understanding is that it is easier to get a reservation there from mid-August through the end of the season.
There are also the intangible or smaller things. One of our ABD guides, Tiffany, had so much knowledge of the area and knew all about the various geothermal features. This made our walks/hikes or time on the bus extremely educational. Or, as we were looking at a geothermal feature, she would pull out a laser thermometer and the kids would have a ball.
Our son is 10 also and he really had a awesome time on this trip. He even swam in the Snake River for a set of dog tags that the kids got if they jumped in.
In terms of the ages of the kids on our trip, there were about 6 or 7 between the ages of 7-10 and another 6 or 7 between 12-15.