Set Up an E-mail Account for Your Child That You Can Monitor

Send an e-mail to your child's new Gmail account. Access the account to see if it arrived, then check to see if it's forwarded to your own inbox, formatted as you've set up. If everything's working, you'll know that any incoming mail to your child will be seen by you. It's not a perfect system; you'll still have to access the Gmail inbox directly to see any mail your child sends, and if your child at some point decides on privacy and changes the forwarding and password, you'll be out in the cold. But if your child is agreeable to your oversight, or you're just using the e-mail to access other services like iTunes, this set-up should work just fine.