Thanks for all the feedback on Wren's bruised legs. I took him to the pediatrician this afternoon and she explained that the main concern with "excess" bruising is low platelet levels. If they suspect low platelets then he would need a blood panel (a full blood draw) which we could do at Children's.
However, she is not sure he needs it. Apparently, his bruises are definitely more than normal but in their location and size they could easily be "normal active boy" bruises. She showed me what to watch for - more larger bruises, bruises in locations that would not be common from falls and also [WARNING GROSS PICTURE] petechia which are a sign of low platelet counts caused by various ominous conditions.
If Wren gets more unusual bruises we will have his blood levels checked, but he seems to be just unfortunate (or prone to accidents) at the moment.