sup_child.erl

child(SupPid, Child) ->                                                                                                                     
    find_child(                                                                                                                             
      Child,                                                                                                                                
      supervisor:which_children(SupPid)                                                                                                     
     ).                                                                                                                                     
                                                                                                                                            
find_child(_, []) ->                                                                                                                        
    {error, not_found};                                                                                                                     
find_child(Child, [{Child,Pid, _,_}|_]) ->                                                                                                  
    {ok, Pid};                                                                                                                              
find_child(Child, [_|Rest]) ->                                                                                                              
    find_child(Child, Rest).