A traumatic injury, the victim will be in severe pain and probably be unable to move the limb. They can quickly go into shock.
Children can avoid breaking bones by following the same safety rules as for falling.
To treat, you may need to straighten the bone. Straightening only should be done when the appendages below the break are blue, cold, numb or paralyzed. Use your handkerchief and gauze bandages to protect any protruding bones. Immobilize the bone by splinting the joint. This will help reduce pain.
Next, support the injured bone with a sling or by bandaging it to an unaffected part of the body. Treat for shock. You may have to carry out the victim or call for medical help. If walking out, carry the victim’s gear.
Read more about day hiking with children in my guidebook Hikes with Tykes.