Arches is very good for a 5 YO (or anybody else, really). Easy to get around by car with lots of easy hikes that a youngster would enjoy. I took my 5 YO there last year.

Getting down into Canyonlands is much more challenging but, OTOH, there are scenic overlooks if you just want to drive up and check it out from above.

The drive from the interstate to Arches along the Colorado River (128) is very scenic. There are a couple of resorts along the way that offer trail rides and whatnot.