Unfortunately, you can not have two SMS students with the same email, since they'll need to be paired to Moodle accounts and Moodle does not support having two accounts with the same email address very well. We recommend that students create their own email accounts rather than use a parents.
Sometimes the district provides student accounts for them to use, but if that is not the case in your situation, additionally there's lots of free mail providers like Gmail.
One alternative option is that, if the parent uses gmail, they can create two "different" emails that go to the same place by adding a "+" sign to the email. So, for example, "[email protected]" and "[email protected]" both go to "[email protected]", but SMS and Moodle treat them like unique emails.
In practice we have heard that this leads to significant confusion though, and eventually students will get their own email as they get older, so it's best to recommend they have their own accounts right off the bat (even if the accounts are parent administered until the students are more mature).