"In OpenDNS' login page it states: "Forgot Your Password." All one has to do is click the link put in my email address and an e-mail will be sent with a link to change the password. Once that's done, bingo, you're in."
The forgot password system relies on the email address tied to your account not being accessible. In 99.9% of cases, the user trying to gain access will not have access to your account's email and therefore will not be able to gain access via the method you describe. I'd recommend using different Mac accounts and avoid having your email program authenticated to every user. Did you know that you can also store your personal passwords in a different keychain with a unique password?
As it's been mentioned, you should consider your email account access an alternative authentication method (via password resets, etc) to any service you sign up with that email from. With access to your email account, you should consider every single service you signed up for to be accessible to your entire family because you have granted access to your identity in the form of your email address.
You will need to either use a private email address or lock out your email account to prevent your son from regaining access to your OpenDNS or other accounts that you are giving him permission to do by sharing your email account.
