Automatic adjustment of items in itemize to fill the slide

In the following MWE, I manually define setlengthitemsep{7em} to define the spacing between items. Is there a nice and reusable way to auto-adjust, so the slide does not look empty from the bottom? It should, however, start from the top to ensure that some animations stay on the same place in the overlays. (e.g. frame 2)

Thanks in advance

documentclass{beamer}
mode<presentation>{usetheme{Madrid}}

usepackage{tikz}
usetikzlibrary{shapes.geometric, arrows}
tikzstyle{process} = [rectangle, minimum width=3cm, minimum height=1cm, text centered, 
text width=3cm, thick,draw=black, fill=orange!30]
tikzstyle{arrow} = [thick,->,>=stealth]

begin{document}
begin{frame}[t]{Frame Title}
begin{itemize}
setlengthitemsep{7em}
item 1
item<+(1)-> 2 
item<+(1)-> 3
end{itemize}
end{frame}

begin{frame}[t]{Frame Title}{}
begin{itemize}

item 1

begin{tikzpicture}[node distance=3cm]

node<+(1)-> (root) [process] {root};
node<.(1)-> (child1) [process, below of=root, xshift=-4cm] {child1};
draw<.(1)-> [arrow] (root.south) -- (child1.north);

node<+(1)-> (child2) [process, below of=root, xshift=4cm] {child2};
draw<.(1)-> [arrow] (root.south) -- (child2.north);

end{tikzpicture}

item <+(1)-> 2
end{itemize}
end{frame}

end{document}