Position a TikZ node in relation to a specific Forest node

Question

Is it possible to avoid hardcoding the \node position in the below MWE and apply horisontal offset of the Forest node to the TikZ node it links to?

The goal is to put the green nodes as close as possible to right above the linked Forest node, but at the same time try not to overlap other Forest nodes. The vertical position will have to be hardcoded.

\documentclass{standalone}
\usepackage{forest}

\begin{document}
\begin{forest} 
for tree={
    draw=black, align=center, l sep=4ex, parent anchor=south, child anchor=north,
    node options={font=\footnotesize, minimum width=14em, minimum height=10ex},
    edge path={
        \noexpand\path[\forestoption{edge}]
        (!u.parent anchor) -- +(0,-2ex) -| (.child anchor)\forestoption{edge label};
    }
}
[Parent
    [SubParent
        [Child1
            [Child11]
            [Child12,name=Child12]
        ]
        [Child2
            [Child21]
            [Child22]
            [Child23,name=Child23]
        ]
        [Child3
            [Child31]
            [Child32]
        ]
    ]
]
%
\tikzset{every node/.style={font=\footnotesize, draw=green, minimum width=14em, minimum height=10ex}}
%
\node[anchor=south,draw=green](Second1)  at (-30em, -15ex) {Second Parent 1}[];
\node[anchor=south,draw=green](Second2) at (15em, -15ex) {Second Parent 2}[];
%
\draw[->,dotted] (Child12) to[out=north east,in=south] (Second1);
\draw[->,dotted] (Child23) to[out=north east,in=south] (Second2);
\end{forest}
\end{document}

Answer 1

You don't actually need the positioning library for this although that is one option. At least, if I've understood what you want to do correctly:

\documentclass{standalone}
\usepackage{forest}

\begin{document}
  \begin{forest}
    for tree={
      draw=black, align=center, l sep=4ex, parent anchor=south, child anchor=north,
      node options={font=\footnotesize, minimum width=14em, minimum height=10ex},
      edge path={
        \noexpand\path[\forestoption{edge}]
        (!u.parent anchor) -- +(0,-2ex) -| (.child anchor)\forestoption{edge label};
      }
    }
    [Parent
    [SubParent
    [Child1
    [Child11]
    [Child12,name=Child12]
    ]
    [Child2
    [Child21]
    [Child22]
    [Child23,name=Child23]
    ]
    [Child3
    [Child31]
    [Child32]
    ]
    ]
    ]
    %
    \tikzset{every node/.style={font=\footnotesize, draw=green, minimum width=14em, minimum height=10ex}}
    %
    \node[anchor=south,draw=green](Second1)  at (Child12 |- 0,-15ex) {Second Parent 1}[];
    \node[anchor=south,draw=green](Second2) at (Child23 |- 0,-15ex) {Second Parent 2}[];
    %
    \draw[->,dotted] (Child12) to[out=north east,in=south] (Second1);
    \draw[->,dotted] (Child23) to[out=north east,in=south] (Second2);
  \end{forest}
\end{document}

Answer 2

Another way is to use the yshift key in setting coordinates. Since your green box Second1 is directly above the Child12 node, you can use

\draw(Child12) to ([yshift=25ex]Child12) node{Second Parent 1};

to draw a line from Child12 to a coordinate 25ex units above it. Including the node operation in the same path allows you to place a node at the latter coordinate. Setting the appropriate styles for draw and node is easy from here on.

Code

\documentclass{standalone}
\usepackage{forest}

\begin{document}
\begin{forest} 
for tree={
    draw=black, align=center, l sep=4ex, parent anchor=south, child anchor=north,
    node options={font=\footnotesize, minimum width=14em, minimum height=10ex},
    edge path={
        \noexpand\path[\forestoption{edge}]
        (!u.parent anchor) -- +(0,-2ex) -| (.child anchor)\forestoption{edge label};
    }
}
[Parent
    [SubParent
        [Child1
            [Child11]
            [Child12,name=Child12]
        ]
        [Child2
            [Child21]
            [Child22]
            [Child23,name=Child23]
        ]
        [Child3
            [Child31]
            [Child32]
        ]
    ]
]
%
\tikzset{every node/.style={font=\footnotesize, draw=green, solid, minimum width=14em, minimum height=10ex}}
\draw[->,dotted] (Child12) to[out=north east,in=south] ([yshift=25ex]Child12) 
  node[anchor=south]{Second Parent 1};
\draw[->,dotted] (Child23) to[out=north east,in=south] ([yshift=25ex]Child23) 
  node[anchor=south]{Second Parent 2};
\end{forest}
\end{document}

Output