So sorry I am just seeing this, now. If it is still helpful, here's how to do it. Set up your own gmail account (like firstname.lastname@example.org, then have the kids use your gmail in this format: email@example.com, where "jdoe" is their username. They can sign up for something like Shelfari using that address and they don't need their own e-mails. Many sites don't allow the plus sign within the address, so it doesn't work for those sites, but I've found it to work for several.
Any e-mail sent to the kids at firstname.lastname@example.org will come to your email@example.com address, so it's best to set up a dummy account that you don't use otherwise, but it works great.