%\begin{tikzpicture}[>=latex, node distance=1cm, on grid, auto,level/.style={sibling distance=60mm/#1}]
\begin{tikzpicture}[>=latex, node distance=1cm, on grid,
auto,composition/.style={yshift=11mm,scale=0.6,diamond,very thin,draw,fill=black}]
\node [rectangle,draw] (root){$Process_{root}$}
child{node[composition] (rc) {}
  child {
    node [rectangle,draw] (a) {$WD_A$}
  }
  child {
    node [rectangle,draw] (s1) {$WS_1$}
  }
  child {node [rectangle,draw] (b) {$WD_B$}
    child{node[composition] (bc) {}
      child {node [rectangle,draw] (child) {$Process_{child}$}
        child{node[composition] (cc) {}
          child{node [rectangle,draw] (c) {$WD_C$}}
          child{node [rectangle,draw] (s2) {$WS_2$}}
          child{node [rectangle,draw] (d) {$WD_D$}}
        }
      }
    }
  }
};

%\node[composition] at (root.south) [label=] {};
%\node[composition] at (b.south) [label=] {};
%\node[composition] at (child.south) [label=] {};
\end{tikzpicture}