From the squid FAQ.
The cache_peer_domain directive allows you to specify that certain caches siblings or parents for certain domains:
# squid.conf - On the host: sv.cache.nlanr.net
#
# Format is: hostname type http_port udp_port
#
cache_peer electraglide.geog.unsw.edu.au parent 3128 3130
cache_peer cache1.nzgate.net.nz parent 3128 3130
cache_peer pb.cache.nlanr.net parent 3128 3130
cache_peer it.cache.nlanr.net parent 3128 3130
cache_peer sd.cache.nlanr.net parent 3128 3130
cache_peer uc.cache.nlanr.net sibling 3128 3130
cache_peer bo.cache.nlanr.net sibling 3128 3130
cache_peer_domain electraglide.geog.unsw.edu.au .au
cache_peer_domain cache1.nzgate.net.nz .au .aq .fj .nz
cache_peer_domain pb.cache.nlanr.net .uk .de .fr .no .se .it
cache_peer_domain it.cache.nlanr.net .uk .de .fr .no .se .it
cache_peer_domain sd.cache.nlanr.net .mx .za .mu .zm
The configuration above indicates that the cache will use pb.cache.nlanr.net and it.cache.nlanr.net for domains uk, de, fr, no, se and it, sd.cache.nlanr.net for domains mx, za, mu and zm, and cache1.nzgate.net.nz for domains au, aq, fj, and nz.
Comments about cache_peer_domain from the example squid configuration file.
# TAG: cache_peer_domain
# Use to limit the domains for which a neighbor cache will be
# queried. Usage:
#
# cache_peer_domain cache-host domain [domain ...]
# cache_peer_domain cache-host !domain
#
# For example, specifying
#
# cache_peer_domain parent.foo.net .edu
#
# has the effect such that UDP query packets are sent to
# 'bigserver' only when the requested object exists on a
# server in the .edu domain. Prefixing the domain name
# with '!' means the cache will be queried for objects
# NOT in that domain.
#
# NOTE: * Any number of domains may be given for a cache-host,
# either on the same or separate lines.
# * When multiple domains are given for a particular
# cache-host, the first matched domain is applied.
# * Cache hosts with no domain restrictions are queried
# for all requests.
# * There are no defaults.
# * There is also a 'cache_peer_access' tag in the ACL
# section.
#
#Default:
# none