Matt, Ive had a chance to camp at Merriam Lake twice, and it is awesome. The trail is in good shape and easy to follow, but it is steep in places. Its a good workout, but not dangerous. Your 9 year old should be fine if you take your time. This is the same trailhead as used for Leatherman, but a different trail. Not sure if its in the Lopez book, but it is in Trails of Eastern Idaho. Ive had decent luck fishing for brook trout there, mostly near the inlet.