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Set your e-mail to recognize the forwarded messages.

Set up your e-mail to flag your child's messages.
Screenshot of Mac OS X Mail
Depending on your own particular e-mail program, you may be able to set up some rules to mark the messages forwarded from your child's Gmail account. In Mac OS X Mail, I've set up a rule to make messages with my daughter's Gmail name in the "To" space appear in my inbox in orange type, so they stand out from my other messages. You might choose instead to have those forwards go into a specified folder. Refer to your software program's instructions for options on flagging incoming mail items.