$$
is defined to return the process ID of the parent in a subshell; from the man page under "Special Parameters":
$ Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the current shell, not the subshell.
In bash
4, you can get the process ID of the child with BASHPID
.
~ $ echo $$
17601
~ $ ( echo $$; echo $BASHPID )
17601
17634