Well it depends on the age of the kid and their experience level. They are like adults that way. You can't expect a kid who isn't used to hiking to do an 8 mile out and back. It's asking too much of them just like it is with an adult.
I have yet to meet a kid that, given an appropriate hike in a good place, does not enjoy it. Just like anyone else some need to be eased into it more than others, and some need more personal attention than others, but they all find some reason to get into it. They key is not interfering with whatever style they have for interacting with their surroundings. Some are purely driving to see what is next. Go, go, go! Others want to stop and examine things every 500 feet.
I remember on Mt Monadnock in NH a few years ago my older son found an orange salamander crossing the trail. He loved that thing and for 30 minutes it was his whole world. A first all I could think about was dwindling sunlight and how many miles we had left. After ten minutes of that, though, I said screw it look how happy he is, I want some of that. I sat down with him and watched the salamander crawl across the trail. I would never have done that if I were by myself and that's one of my favorite hiking memories of the last few years.
