I`m doing Yellowstone next year. the first 2 days are going to be 10-11 hours each. the 3rd day will only be 4 hours and I`ll stop and sight see on our way into Yellowstone. I do 11+ hours quite often. we went to Glacier NP back in 05 and did a few 10+ hour days and my kids were 3 and 4, granted we were in a conversion van. and no one had any problems with the drive. driving is relaxing to me.
We`re only doing 4 days in Yellowstone then 2 down in the Tetons. then seeing Rocky Mtn NP in CO. on our way back.
Maybe add in The Badlands and Black Hills as a good stopover for a few days, then head into Yellowstone through Cody WY. with 10 days after driving I would do 3 or 4 in the Black Hills and 6 or 7 in Yellowstone. I actually split the Blackhills up as a stopover on my Glacier NP trip. it broke it up nicely.
