Question

Why do parents and doctors get worried if a child gets head or spinal injury

Answer 1

Let's say you are in a sport and you have a back injury and its serious that can be the end of your career for sports ,you may never recover,you can becomes paraylzed

Answer 2

If somebody gets a head injury, they may be susceptible to a concussion, brain swelling, bleeding within brain tissue, skull fracture, tissue damage, etc. 
A spinal injury could cause things like, paraplegia, motor skill loss, nerve damage, etc.