Are your files in source control anywhere? I'll happily do it for you.
But, short version, you don't add it to BasiliskII executable. The information to enable the About box is in the other files in the app package. You'd add them to /Applications/Parent.app/Contents/Resources/Child.app/... if you get what I mean?
Right click the app and Show Contents, there is the minimum contents in this demo App to show you how to do the thing you wanted.
Summary... (* denotes essential requirement for About box customisation)
* NSHumanReadableCopyright (string) "Created by..."
* CFBundleShortVersionString (string) "1.0"
* CFBundleVersion (string) "1234a"
* CFBundleExecutable (string) "applet"
CFBundleName (string) "AppName" [app name of parent]
* applet (launches main.scpt)
* applet.rsrc (contains information required by the applet binary, see here
* Credits.rtf (rich text file containing credits text, can contain hyperlinks)
description.rtfd (leave this, no need to edit it)
Classic.icns (classic happy mac iconset)
Finder.icns (classic finder mac iconset)
* main.scpt (this should launch the shell script that in turn launches BasiliskII)