The key is to not push them too hard. Give them small tasks that do not require large hikes in or full day efforts. Today, I brought my daughters out to Scott's Drop, where we cleared many downed trees that made the trail impassible. While I did the heavy lifting, I set them off trimming smaller brush with loppers and handheld clippers. After several hours, we wrapped it up, climbed back in the truck and headed home. Both of the girls told me they really liked our outing and would love to do it again.
Which is about as nice a thing as I could hear on this Christmas Eve.