Just add the following line tou your squid config.
cache_peer <your_parent_cache_ip> parent 3128 3130 no-query default no-digest
OPTIONS WHICH AFFECT THE NEIGHBOR SELECTION ALGORITHM section in Squid.conf for explanation of this.
After sthis your proxy will fetch static documents without any errors, but if you want to fetch something containing ? or cgi-bin, it will try to bypass the parent cache. To prevent it from doing this add the following line:
never_direct allow all