Yellowstone!
If I had to choose with small children, I would go to Yellowstone rather than the Canyon.
(I have whitewater rafted the length of the Canyon and have spent weeks at Yellowstone so I am more than familiar with both.)
Kids will be much more interested in geysers, buffalo and bears than in a hole in the ground, no matter how impressive to adults.
Formerly posting as "littleblackdog"
Martha, Allen, & Blackjack
2006 Chevy 3500 D/A LB SRW, RVND 7710
Previously: 2008 Titanium 30E35SA. Currently no trailer due to age & mobility problems. Very sad!
"Real Jeeps have round headlights"