Unless you actually have to carry a child your hike will not be very physically exerting. My husband hikes alone for the physical challenge of it and then hikes with our kids for entertainment. Hiking with your children is a learning experience for both you and your children. You learn to slow down and enjoy things as they come. Sore feet make for grumpy kids.
Some people like to view plant life.
There is no better way to see unfettered plant life than out on a lonely trail. Other folks like to view Native American rock art and ruins. The more secluded the area, the more likely you are to find quality remains of Native American culture. If you are seeking bird or animal life you may want to wait until your kids are old enough to remain quiet for a while - a young kid will never keep quiet enough for animals to be seen. Try to set a good example for your kids.