hi,
i too have an arachnoid cyst and have a vp shunt fitted due to this, ive had 3 fitted since i was 14 months old.
during those 17 years i have smacked and banged my head on numerous occassions, even played football and headed the ball at oncoming power, i havent had any problems with the shunt after banging my head that im aware of as they are very strong =]
of course this is just reassurance...but if there are any of the symptoms of shunt malfunctions...like irritability, headaches, convulsions,sickness the size of the head growing or the baby not seeming like himself then please get him checked out because of course these shunts are invincible afterall.
i hope everythings and well
take care x