Little girls like poop hikes, too. You are getting really good advice in this thread, OP, and I don't have a lot to add. In my experience with my granddaughters, now 9 and almost 6, the hike has to be "interesting" (wildlife, waterfalls, other hikers, views, a little climbing), and not too long. You can start with short walks in the backyard -- look for bugs and flowers, listen for birds, and so on. If you have a magnifying glass, take that along ,too.
Once your niece can hold her head up well (maybe 6 months of age, but it varies), she will probably enjoy riding in a child backpack. She'll be able to see a lot from her lofty perch, and you'll know she's safe up there.