%\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){$VirtualRoot$}
child{node[composition] (rc) {}
  child {
    node [rectangle,draw] (d) {$D$}
  }
  child {
    node [rectangle,draw] (a) {$A$}
  }
  child {
    node [rectangle,draw] (b) {$B$}
  }
  child {
    node [rectangle,draw] (c) {$C$}
  }
};

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

\path (d) edge [red,pre,open triangle 45-] (a)
      (a) edge [red,pre,open triangle 45-] (b) ;

\end{tikzpicture}