As a young child and even today as a 48.5 year old man, I wiggle my fingers in front of my eyes. I have done it all my life. Only recently did I figure out that what I was doing was helping myself focus as I am slightly ADHD. I imagine my fingers being as "fire" consuming the page in front of me, as I read. I did this most often as a child, while trying to read, because my mind was always racing and I was distracted from what I was trying to read and comprehend. I usually did it and do it when I am by myself, as I am very self-conscious of the activity when others are around me. I have been told that I will never have to worry about arthritis in my hands, because I do this quite often and it keeps my joints moving. For that I am thankful. Sometimes, when my fingers do get stiff, I start wiggling them and in no time, they are as limber as all get out and no pain at all.
Your child may be compensating for a slight deficiency in "attention keeping and focusing" or just burning off nervous energy. Although early on in my life, I saw it as a curse, because I was always getting into trouble by "adults" who did not understand what I was doing, but over the years I have not only accepted the activity as a necessary part of myself, but have come to see it as a blessing in my life, as it is now a power I possess and that no one but GOD or myself can take away from me. Believe it or not, it now adds a particular "excitement" to my life now as anytime I am having a challenge in staying focused, I just apply the technique and in no time I assimilate information that it takes many people much longer to assimilate.