sounds like you forgot to add the host name mapping for "hostnameofchildsite.com" to the child site. It is redirecting fine but the host name is not assigned to the child site so the default site is shown. You need both the preferred host name to be set and the host name must be in the host names attached to the child site in the site settings of the child site from Master Site > Administration > Sites click on the child site to get to site settings and add the host name in the host name tab.