My experience is that most young children (<7) don't fully appreciate the beauty in nature... so within a mile you start hearing comments like "I am tired of walking!", "I am bored!", "When are we going back to camp?", etc. We have found having some sort of supplemental activity really helps kids enjoy the time. Examples of this are: walking on interpretive trails, taking lead nature walks, making it a treasure hunt (e.g. looking for interesting items you know the kids will be able to find).
We have found that our daughter wants to carry her pack just like her parents carry their packs. On day hikes she carries her own water, a camera, a couple of toys, a snack, plastic plant & bird identification cards, and a nature notebook & crayons which she draws pictures of interesting discoveries. On overnights she carries everything except her sleeping bag and pad which are too much volume for her pack.
Other resources:
Scouting
Typically boy scout backpacking is the opposite of ultra-light backpacking. Everything including the kitchen sink (or at least a dutch oven to make cobber) is taken. Gear is typically extremely heavy-duty to survive the riggers of younger boys. Some good references for scouts include: