Node that creates another one which reads its values dynamically

I’m trying to understand how the expansion of a pgfextra which in turn is inside an append after command is done. My idea is to read some value from a pgf key when the append after command code is executed, but I’m afraid the code is being expanded on the style definition.

documentclass[12pt]{standalone}

usepackage{tikz}
usetikzlibrary{calc}
usetikzlibrary{arrows.meta}

begin{document}
begin{tikzpicture}

pgfkeys{
    /my label/.initial=initial,
}

tikzset{my node/.style={
    draw=red,
    minimum size=1cm,
    node contents=#1,
    append after command={
        pgfextra
        draw [-Implies, double, shorten >=2ex] (tikzlastnode.south) -- +(0, -1cm);
        path [use as bounding box] (tikzlastnode.south west) rectangle ($ (tikzlastnode.south west) + (0, -1cm) $);
        pgftransformshift{pgfpoint{0pt}{-1.5cm}}
        pgfnode{rectangle}{base}{pgfkeysvalueof{/my label}}{}{pgfusepath{discard}}
        endpgfextra
    }
}}

fill (-1, 1) [white] rectangle (1, -2);
node at (0, 0) [/my label=label, my node=1] {};

end{tikzpicture}
end{document}

What I get

vs what I’m trying to achieve

What I eventually want to do is to draw two nodes like the image from above that can be fully referenced later by only specifying one node, for example:

% this create a node with `name` and `node contents` K and a node below with `name` and `node contents` V.
% the argument V could be set here or read dynamically from a pgf key)
node [key=K, value=V] 

draw [->] (K.north) -- +(0, 1);
draw [->] (V.south) -- +(0, -1);

Any way to achieve this or must I specify two nodes manually?

documentclass[border=2pt]{standalone}
usepackage{tikz}
usetikzlibrary{calc}
usetikzlibrary{arrows.meta}
% KeyValue[options for the upper node]{position}{name of upper node}{name of lower node}
newcommandKeyValue[4][]{%
  node[draw=red,minimum size=1cm,#1] (#3) at (#2) {3};
  draw[-Implies,double] (#3) -- +(0, -1cm) node[below,inner sep=1pt](#4){#4};
}
begin{document}
begin{tikzpicture}
  KeyValue{0,0}{1}{label}
  draw [->] (1.north) -- +(0, 1);
  draw [->] (label.south) -- +(0, -1);
end{tikzpicture}
end{document}