I think this is what you are looking for: You want to insert tikz code inside the braces that define the node content. To do this, create a tikzpicture (with begin{tikzpicture} and \end{tikzpicture} within the braces. Another way, shorter, is to use the \tikz command (as in my example). Keep in mind that the coordinates defined within this embedded picture are not linked to the initial tikzpicture, they are local.
The example is
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\begin{tikzpicture}
\node[rectangle,draw,inner sep=5pt,fill=blue] (example) {\tikz{\draw[fill=red] (0,0) circle[radius=1cm];}};
\end{tikzpicture}