While the Pine Creek Trail is lovely, you're not going to find playgrounds, ice cream, or just about any other amenities other than the campground mentioned. The GAP is not flat and as someone mentioned, nothing in the way of amenities from the Eastern Continental Divide to Frostburg (and a nasty hill from the trail to Frostburg), or from Frostburg to Cumberland. I do not think trails in the DC area would work for a 5 year old. Some of the trails are very busy, and you would inevitably have to ride on some urban streets at times. I've never ridden the Erie Canal route, but it sounds like it may be your best bet.