It's a slightly different question but you should be able to use the same technique as described here, i.e. use the api in your message template to look up the info:
Parent's emails should be visible from the child's "Relationships" tab without actually clicking through to the parent.
Alternatively, the Relationship Block extension allows you to put the parent's info on the summary screen. Not ideal, but it saves you a click.
After that, I think you're looking at commissioning an extension, or a modification to an ...