mhh. i dont think it will be run if you play on same time because your game is linked to your origin login. if you login too you will got a error like "ID is still used" or something else.
but you have to think about a othe point. if your nephew is going to do some shit on this like cheating. he got a ban and than this will be linked to your account too.
never know how some "kids" think and handle.
so i would say buy a new copie of bf2. must be around a tenner or so. so you on the save site.