Setting cache hierarchy

Squid
Hi squiders,
I have this problem:

We have 1 big cache for all customers. They are using it with browsers.
But some customers want use it as parent proxy(they have squid too).

1.How I have to configure our proxy that customers can fetch?

2.How configure customers caches to fetch and SAVE fatched files to
their local caches? Not only fetch! ^^^^

Best regards

-- 
 ---------------------- Martin Svoboda ---------------------------