Yes, a 30' TT will be too long for most of the CGs inside the park. Even in Fishing Bridge, you may find backing into your site to be quite an adventure, due to trees and other solid items across the narrow CG road from the campsites. (Our TT is 27' and it was tough fitting into our site in FB.) If you want to stay at a CG in the park besides FB, you might look into Bridge Bay; some of the non-wooded sites in there might accommodate your rig better than Canyon's will.
Another vote for Grizzly. Although you can't beat the location of Fishing Bridge, Grizzly is a more comfortable CG (IMO), and still close enough to the park's western border to be easy to get in & out of the park. It's usually where we stay.
You will probably spend almost all the daylight hours away from your campsite, given how much there is to see & do. You know your kids, though, and if they are likely to want to spend a day, or part of each day, at the campground, then opt for the KOA (the original one, closer to West Yellowstone) as they have an indoor pool. Otherwise, Grizzly or Fishing Bridge should fit the bill.