I was in Moab last july, it was uncomfortably hot. 45 degrees and dry as hell. A 2 kilometer hike to see some of the arches was hard to do. I'm not saying not to go if your determined. Just be prepared to sweat your bag off. I've heard its much better in spring and fall. We were just touring through on a long road trip, I plan on going back one day for some awsome trails but I'l definately do it anytime but mid summer.
Takes about 17 or 18 hrs to drive from calgary so probably 2 days with kids. You could spend a week there easy doing not only trails but some easy sight seeing as well.