Planning to do a driving tour, stopping at the pullouts, is important to have as a backup plan when traveling with a young child, if nothing else. At Arches there are lots great pullouts where you can actually see a fair bit from the side of the road. There are also lots of short/easy trails, where a 1/2 mile or mile of fairly level walking takes you to even better views. Canyonlands is tougher because most of the trails are at least a few miles long and traverse rougher terrain.