you can create a node at each end of the lines and then connect these nodes. by adjusting the minimum size of node you can improve aesthetics.
(sorry for my google english)
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows,decorations.pathmorphing,backgrounds,positioning,fit,petri,calc,shadows}
\begin{document}
\begin{tikzpicture}[
parent/.style={%
rounded corners,
thick,
draw=red!75,
fill=red!20,
thick,
inner ysep=2pt,
inner xsep=2pt,
minimum width = 4cm,
minimum height = 1.5cm,
align=center
},
child/.style={%
rounded corners,
thick,
draw=blue!90,
fill=blue!35,
thick,
inner ysep=2pt,
inner xsep=2pt,
minimum width = 4cm,
minimum height = 1.5cm,
align=center
},
grandchild/.style={%
rounded corners,
thick,
draw=green!90,
fill=green!35,
thick,
inner ysep=2pt,
inner xsep=2pt,
minimum width = 4cm,
minimum height = 1.5cm,
align=center
},
line/.style={%
semithick,
->,
shorten >=1pt,
>=stealth'
},
call/.style={%
blue,
semithick,
->,
shorten >=1pt,
>=stealth'
},
return/.style={%
red,
semithick,
->,
shorten >=1pt,
>=stealth'
}]
\node[child] (child) {Child};
\node[parent] at (-6,3) (parent 1) {Node 1\\I have manually\\connected this one};
\node[parent] at (0,3) (parent 2) {Node 2\\I have manually\\connected this one};
\node[parent] at (6,3) (parent 3) {Node 3\\I have manually\\connected this one};
\node[parent] at (-6,-3) (grandchild 1) {Node 4\\I have manually\\connected this one};
\node[parent] at (0,-3) (grandchild 2) {Node 5\\I have NOT manually\\connacted this one};
\node[parent] at (6,-3) (grandchild 3) {Node 6\\I have NOT manually\\connacted this one};
%draw three lines from each parent to each child
\draw [line] (parent 1.south east)node[above left](p1){} -- (child.north west)node[below right](c1){};
\draw [line] (parent 2.south)node[above](p2){} -- (child.north)node[below](c2){};
\draw [line] (parent 3.south west)node[above right](p3){} -- (child.north east) node[below left](c3){};
%draw three lines from each parent to each child
\draw [line] (grandchild 1.north east)node[below left,minimum size=2em](p4){} -- (child.south west)node[above right,minimum size=2em](c4){};
\draw [line] (grandchild 2.north)node[below,minimum size=2em](p5){} -- (child.south)node[above,minimum size=2em](c5){};
\draw [line] (grandchild 3.north west)node[below right](p6){} -- (child.south east)node[above left](c6){};
\foreach \nn in{1,2,3,4,5,6}{
\draw [call] (p\nn) to [bend right=15] (c\nn);
\draw [return] (c\nn) to [bend right=15] (p\nn);
}
\end{tikzpicture}
\end{document}!
Here's something like the second option (the node does exist).
There are only two changes to your MWE, in attribute/,style:
The xshift needs adjusting
The anchor point you need can be expressed as an angle
Sorry: I pass on the bonus.
Here it is -- I leave final adjustments to your taste:
\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usetikzlibrary{trees}
\tikzstyle{every node}=[draw=black,thick,anchor=west, minimum height=2.5em]
\begin{document}
\begin{figure}[!htb]
\resizebox{\linewidth}{!}{
\begin{tikzpicture}[
criteria/.style={text centered, text width=2cm, fill=gray!50},
attribute/.style={%
grow=down, xshift=0cm,
text centered, text width=2cm,
edge from parent path={(\tikzparentnode.225) |- (\tikzchildnode.west)}},
first/.style ={level distance=8ex},
second/.style ={level distance=16ex},
third/.style ={level distance=24ex},
fourth/.style ={level distance=32ex},
fifth/.style ={level distance=40ex},
level 1/.style={sibling distance=10em}]
% Main Goal
\node[anchor=south]{SuperLongTitleWithNoMeaning}
[edge from parent fork down]
% Criteria and Attributes
child{node (crit1) [criteria] {Criteria1}
child[attribute,first] {node {Attribute\\Number1}}
child[attribute,second] {node {Attribute2}}
child[attribute,third] {node {Attribute3}}
child[attribute,fourth] {node {Attribute4}}
child[attribute,fifth] {node {Another\\Attribute}}}
%
child{node [criteria] {Criteria2}
child[attribute,first] {node {Attribute1}}
child[attribute,second] {node {Attribute2}}
child[attribute,third] {node {Third\\Criteria}}
child[attribute,fourth] {node {Longtext-\\criteria}}}
%
child{node [criteria] {Criteria3}
child[attribute,first] {node {Attribute\\two lines}}
child[attribute,second] {node {Attribute2}}
child[attribute,third] {node {Attribute3}}}
%
child{node [criteria] {Criteria4}
child[attribute,first] {node {Attribute1}}
child[attribute,second] {node {Attribute2}}}
%
child{node [criteria] {Criteria5}
child[attribute,first] {node {First\\Attribute}}
child[attribute,second] {node {Attribute2}}
child[attribute,third] {node {Third\\Criteria}}
child[attribute,fourth] {node {Longtext-\\criteria}}};
\end{tikzpicture}}
\caption{This is a nice tree.}
\end{figure}
\end{document}
Best Answer
Replace
-latexinedge from parentstyle withlatex-.