Sometimes younger children exhaust themselves by running as soon as they hit the trail. You then end up carrying them most of the way back or quitting the trail early. Encourage them to pace themselves.
Don’t hike on days when the child clearly needs sleep. If a child obviously is tired, don’t force them on a death march.
Should they tire on the trail, you’ll need to carry their gear. Presuming they’re young, you might want to place them in the baby carrier for the rest of the trip.
Kids’ energy levels also vary from day to day. Maybe last week your child walked three miles with no problem, but today after a half-mile he’s dragging. Whenever you notice a sharp decline in energy in your children, it’s time to turn around. Tired kids are more likely wander off, to fall, or otherwise make bad decisions leading to injuries.
Depending on their age and size, they will have to hike back to the vehicle, of course.
Read more about day hiking with children in my guidebook Hikes with Tykes.