A couple of thoughts:
1. Kids like to carry gear. A five-year old can carry some of his gear and, say, the camp stove. That's a light, but important piece of gear, involves fire, etc., so the kid will feel important and be interested in packing. Also, a flashlight is a real treasure; a pocket knife for older kids.
2. My kids, at the age of five, often lacked the mental stamina to hike distances that were easily within their phyical capacity. This isn't a personality defect, it's just part of a kid's development. Once a kid "gives up" mentally, it's very difficult to get them moving at more than a snail's pace.
3. I found the solution was (1) to avoid mentally challenging obtacles like long climbs or hot weather, (2) take relatively frequent breaks (15 min per hour) and (3) bring a peer/buddy. As an extreme example, my youngest son could literally hike all day if his cousin came along, even a very young age, but by himself could scarcely go a mile.
4. Also, go someplace with some kid friendly features: a small creek, a cave, some cool rocks to climb, a pond with salamanders.