\documentclass{article}
\usepackage{tikz}
\usepackage{xepersian}
\begin{document}

\begin{center}
\begin{tikzpicture}
\node {$b$}
child {[fill] circle (2pt)}
child {[fill] circle (2pt)};

\end{tikzpicture}


\end{center}
\end{document}