My 4yr old has a small red spot on the inner side of the arm. not raised, not itchy. looks like a bruise but too small to be ?

Possible bite.: Depending on how long it has been there, it could still be a bruise, although if it is bright red, it sounds like a broken capillary (small blood vessel) or possibly spider bite. To be sure, you should show his doctor. If there are many of them appearing, you should call his doctor and/or take him to the emergency room ASAP, as that could be a sign of a more serious illness.