7 answers
You need a returl parameter as part of the request to get the child object page - the only way to do this outside of VF at the moment is through a custom new button that opens the standard child page and passes the returl parameter. You'd also need to pass any information required from the parent through url hacks: http://raydehler.com/cloud/clod/salesforce-url-hacking-to-prepopulate-fields-on-a-standard-page-layout.html