Hello Jim.
You can't but you can prevent your kids from accessing your mail and all other information/data in your Home folder/directory by creating a separate single user login account for all your kids (without admin privileges) or a separate user login account for each child/family member without admin privileges. Disable automatic login and enable Fast User Switching which makes the process of switching between multiple computer login accounts a fast and easy process.
With automatic login disabled, no one else can access your login account during a computer startup, restart or via Fast User Switching so ALL data in your Home folder/directory will be kept private, separate and protected from anyone else that uses the computer and the same for all other user login accounts. The Home folder/directotry for each user's login account is kept separate and protected from other users.
OS X is designed for multiple users in this way.
Before stepping away from your Mac, simply log off from your account.
http://www.apple.com/macosx/features/family/
