I am going to link you to a few sites that may help your knowledge base in the long run. because even if you have seen these you could use a refresher. (not to mention I could have linked you the holy grail but you asked me not to so I will keep that to my self.)
but to answer your questions is you need to establish a reference to the variable in the child script that you need. more so then that you need a reference to the class that the variable your want is in.
and there are many multiple ways of doing it and I have yet to learn about which one is right and which are less right.