Furnishing your child with a backpack of their own not only provides an easily accessible space for any nature trinkets your kiddo picks up along the way, but also teaches the concepts of packing and organizational skills, as well as the idea that the whole family shares the load. My son Canaan has quickly learned which toys are conducive to hiking (i.e. fit in the pack along with water bottle, food, camera), and which need to be left at home.
There are kid packs for every budget, but keep in mind that you do get what you pay for, especially when it comes to gear that will be used and abused on the hiking trail. Our family has had a lot of success with the Osprey Jet 18 pack: Designed for 5- to 10-year-olds, it’s a tad too big for my three-year-old son, but he likes carrying it anyway. Plus, when he gets tired of it, it's small enough to throw across my own pack.