Professional :: Loaded SWF Runs Code On Parent Timeline?

I have a parent movie that contains several buttons that load several child SWF files.The parent timeline has only 1 frame, and in that frame all the ActionScript (version 3) for the parent movie is imported with a single statement:
 
include "script.as"; 

The problem is that when a child movie loads, IF the child contains ANY ActionScript calls, it also somehow runs the code in "script.as", which accesses many elements of the parent movie which are not present in the child. This produces "NULL object reference" errors which crash the script:
 
TypeError: Error #1009: Cannot access a property or method of a null object reference.If I remove all ActionScript calls from the child's timeline, then the child doesn't run the parent's code - But if place so much as a stop(); call in the child timeline, then the child runs the parent's code after being loaded in.
 
I originally wrote this script for another project, which works fine, and then copied and pasted it to this one, so I don't understand why it's not working the same way. Earlier on I suspected the trouble might be from re-saving my current project as ActionScript 3, when before this upgrade was saved as ActionScript 2. So I went back and re-created each file in fresh AS3 FLAs, but it didn't solve the problem.The child SWFs are loaded with this code from "script.as".

var ldr:Loader = new Loader();    addChild(ldr);    var url:String = "child.swf";    var urlReq:URLRequest = new URLRequest(url);    ldr.load(urlReq); 

Also in "script.as" is a simple trace call at the top of the file:
 
trace('script.as is running'); 

When de-bugging the file, the message 'script.as is running' appears in the output panel when each child SWF is loaded, which tells me that the child files are running the parent's code. Is there any way to prevent this from happening?