Making a local geological field trip an interesting and educational event for children?
You live in an very good state for interesting geology, rock collecting, etc.  In regards to looking for things the kids can collect, view and otherwise analyse the PDF, "The Collectors Guide to Rocks, Minerals, and Fossil Localities of Utah" would be a good start to your research.  The publication is from the Utah government and covers a large number of areas in the state.  Over 30 years ago I made a trip to the Caineville just east of Capital Reef National Monument and collected pelecypods, they where laying on the soil with the occasional gypsum spars.  I believe the area is in the document I suggest you read, I doubt I could find it now but there was lots to find in the area in general including some evaporative type minerals, agates, and fossils.  

You also have the Great Salt Lake, some areas would be good to collect and view brine shrimp, protozoa, and algae with a microscope, many would be easily viewable with a dissecting microscope.  You could also discuss the geological history of lake Bonneville of which the Great Salt Lake is but a remnant.  

Finally the Gem Trails of Utah guide can provide some sites where collecting might be appropriate, though this publication is getting old and doesn't focus much on geology.

The land status and access to these areas change frequently and sometimes it is difficult to locate the sites, expect some failures in your adventures.  I would also suggest that you field check the surrounding areas because published areas often get damaged by careless individuals.  Finally, would also suggest you follow leave no trace principals as much as possible within a collecting trip, the scenery and wildlife in many of these areas are really impressive and it is a shame to see it all dug up by collectors.

As a final thought most states have some form of Geological Road Maps, there are some listed on this government site.