H.B.
Funny that you asked about the yellow bus tour! My daughter is at Yellowstone right now vacationing with my parents and two nephews. They are planning on riding the yellow bus tomorrow and my daughter is so excited. I'll try to remember to tell you what she thinks about it after I talk to her tomorrow.
On a side note...we did YS when I was little and we (me, mom, dad, sister, brother, gmom, gpop...and our dog) had so much fun! We rented a nice RV and stayed in the campgrounds. I have some great memories and I know your kids will love it. The things I remember the most are Old Faitful, the Grand Tetons and just hanging out at the campgrounds and hot springs.
Have fun!
Update: Just spoke with my DD today! I miss her! Anyway she LOVED the bus ride (they did the all day 11 hr tour. She saw lots of wildlife but her fav was a baby bison. She got that high pitched little girl voice "Mommy it was sooo cuuuuttte!" I know she is having fun!