We got married in the Smoky Mountains, and my favorite hike was Chimney Tops. The scrambling at the end might be a bit much for 3 - 5 year olds, though. My daughter is just 1, so I don't really know what kids are capable of at that age. It's still a great hike, though. Porters Creek was also a very nice hike.
And Cades Cove was cool, but I hear the traffic can be crazy there.