\documentclass[12pt]{standalone}

\usepackage{tikz}

\tikzset{real edge/.style={solid,very thick}}

\begin{document}
\begin{tikzpicture}

    \scoped[every node/.style={solid,thin,circle,draw}]
    \node {1}
    child[real edge] {node {2}
            child[real edge] {node {5}}
            child[real edge] {node {6}}
        }
    child[real edge] {node {3}}
    child[real edge] {node {4}};
\end{tikzpicture}
\end{document}
