It seems that shivering or similar involuntary spasms are quite common among young children. It is not a sign that the child has neurological problems, although I would suggest that you visit your doctor to rule it out. This website is full of similar situations, most kids seem to either be doing the movement on purpose or it will go away on its own.
Check this link out I’m sure it will give you piece of mind. Although you may never get a full explanation of just why your granddaughter is shivering that will just mean that there is nothing significantly wrong.
http://parents.berkeley.edu/advice/babies/tremors.html#9s
