How can I use \linebreak inside a node, in tikz?

There is a much simpler and more elegant solution! From the TikZ manual § 17.4.3 Text Parameters: Alignment and Width for Multi-Line Text:

\node[draw, align=left] {This is a\\demonstration.};

Key here is that you must use the align option, with the parameter that you want.

You could use a single-column tabular to achieve line breaks:

\documentclass{article}
\usepackage{tikz}
\begin{document}
\tikz \node {\begin{tabular}{l}
    start \\
    another \\ 
    stop
\end{tabular}};
\end{document}

Also macros like \shortstack{start\\another\\stop} are useful. I also always add a \strut in every line to ensure a constant line skip.

You can also put a \parbox around your node text:

\documentclass{article}
\usepackage{calc,tikz}
\begin{document}
\tikz \node {\parbox{\widthof{another}}{
    start \\
    another \\ 
    stop}};
\end{document}