Thanks Rivergoer, I'll def hit up Twilight.
Looking at the metrics closer, I doubt I'll do Cathedral in the Desert. The opportunity costs (travel time, gas money, missed things closer to basecamp, etc) outweigh the benefit. Researching it, I think the Escalante arm deserves a couple days alone for another trip. As far as the Cummings Mesa hike, 10 miles might be a bit long for a 9 and 10 year-old... but we will search for the moki steps there. Any other good "hikes" in that general stretch worth looking into, maybe in the >5 mile, >800' el gain range?
On a side note, the snowpack combined with the cooler than average temps means there's a good chance I'll have to shelve the family Colorado trip in July! I was looking to do a few of the high passes in the San Juans but I'm not sure they'll be open by then

I better get a Plan B going...
A few good hikes sort of near Dungeon would be:
1. West Canyon. It's about 7 miles to the mouth of the canyon, then another 6 to the very end at current lake levels. From there, the hike is one of better ones with a little adventure for kids. It's a pretty hike between the tall Navajo Sandstone walls, criss-crossing a creek, and roughly 3 miles to where it really narrows to a near slot where it gets exciting. You might need to get a little wet through the next 100 yards, then it opens briefly before getting to the "swimming pool", which is about 120 yards of swimming (or chest high water) in a slot before emerging on the other side (into the sun to dry off). The water is cold, but it's fun! That's about a 6-7 mile round trip with an elevation gain of next to nothing, maybe 150 feet.
2. Grotto Canyon. Only a mile uplake from Dungeon Canyon... It's a short but pretty box canyon, maybe 1.5 miles each way from the lake, maybe a 400 foot gain.. really fun little hike with nice views...
3. Dry Fork Rock Creek. About 4-5 miles from Dungeon, just follow the creek past the lake into an area that eventually follows a shallow slot for a short time. Makes for a nice out and back hike of any distance you like...
4. Mountain Sheep Canyon. About 5-6 miles from Dungeon Canyon. You might be at just the perfect lake level for Mountain Sheep. The entrance has a lot of submerged rock hazards, but once into the narrow part you ought to be okay...it gets pretty narrow while you're boating to the end, maybe down to 15 feet in places, but there should be a landing... the hike gets into a nice slot and you ought to be able to get at least a mile or two without much trouble, but there will be some boulder hopping and you'll get wet...
5. Hole in the Rock. This is maybe 28 miles north of Dungeon, so pretty far, but a good hike, challenging for kids but doable. It might be only be a mile up and a mile back down, but it's steep--something like an 800-foot gain in that one mile. Great views from the top... best done in the morning to avoid the direct sun of the mid-afternoon, unless its a cool day...
Reflection Canyon and Llewellyn Gulch are also nice hikes in pretty canyons, closer than Hole in the Rock, but still not that close to Dungeon (17 miles to Reflection, maybe 22 to Llewellyn)...