They would have to know all your log-in information for the email account you use. It is possible for a device to be logged into multiple email accounts at the same time (I have 2 on mine) so theoretically a child could be using their own iCloud email address and be logged into your account at the same time, but that has to be deliberate, and as I said, it would require knowing your password.
Some of the details really depend upon which services you use for e-mail and if you are using iCloud at all for which I am not making the assumption you do.