How to use different preamble in subfiles root file and child file

Question

I have both \maketitle \tableofcontents in the root file and child file when using subfiles package. When I compiling the child file, it works OK. But when I compiling the root file, the title and author information and an extra 'Content' are also appeared. I do not know how to erase these informations.

I tried to use macros to control the actions. But it does not work very well.

I have several puzzles.

1.Does the new definition of the variable in child file erased its value transfered from root file?

2.Is there a command to test if a variable here like multiple has already been difnied?

3.When debugging latex, how to output the value you definied in latex?

4.Are there any other graceful ways to achieve my aim?

Thank you!

Here is the root file named root.tex

\documentclass[a4paper, 12pt]{article}
\usepackage{subfiles}

\newif\ifmultiple  %\multipletrue means called by main file
\multipletrue

\author{Ct586}
\title{How to use subfiles}
\date{\today}

\begin{document}

\maketitle

\tableofcontents

\subfile{child}

\end{document}

Here is the child file named child.tex:

\documentclass[Dynamic.Network.Embryonic.Development.tex]{subfiles}

\newif\ifmultiple  %\multipletrue means called by main file
%\multipletrue
\ifmultiple
    \begin{document}
\else
    \author{Ct586}
    \title{How to use subfiles}
    \date{\today}
    \begin{document}
      \maketitle    
      \tableofcontents
\fi


\section{child}
child

\end{document}

Answer 1

set \let\tableofcontents\relax after the main one:

\RequirePackage{filecontents}
\begin{filecontents*}{child.tex}
\documentclass[main.tex]{subfiles}
    \author{Ct586}
    \title{How to use subfiles}
    \date{\today}
\begin{document}
\maketitle    
\tableofcontents

\section{child}
child

\end{document}
\end{filecontents*}
\documentclass[a4paper, 12pt]{article}
\usepackage{subfiles}
\author{Ct586}
\title{How to use subfiles}
\date{\today}
\begin{document}
\maketitle
\def\author#1{} \def\title#1{}
\tableofcontents
\let\tableofcontents\relax

\subfile{child}
\end{document}