Interested in taking about 10-12 days, starting in Las Vegas and driving to Grand Canyon, southwest corner of Colorado and ending up in Denver. Looking for route suggestions and places to stay/things to do/sites to see along the way. Will be traveling with school age children in 3 car caravan.
Things to see and do
There is more to see and do between Vegas and Denver than you could possibly fit into 12 days depending upon the exact route you choose. Therefore, you will have to make choices based on the preferences of your group.
Going south round the Grand Canyon you would see the Hoover Dam, travel on historic route 66 then see the South Rim of the Grand Canyon (the most accessible/popular) travel up to Page & Lake Powell.
Taking the northern route around the Grand Canyon you could see Zion and Bryce National Parks, the north rim of the canyon before getting down to Page and Lake Powell.
From Page you can go through Monument Valley or Four Corners into Colorado to see Mesa Verde National Park and either go north through Utah and see Canyonlands and Arches Nationa Parks or go north through the rockies to see Silverton and Ouray (maybe ride the classic steam railway between the two). Then see the Black Canyon of the Gunnison and either go east to Canon city, see the Royal Gorge before heading north through Colorado springs to Denver. Or go north through Leadville and loop through Rocky Mountain National Park before dropping into Denver.
As I said you will not get all of this into 12 days. Study the maps and make your choices, it should be an excellent trip.
Hope this helps.