When you check your email look to see which emails are Read and Unread. You will know which ones you read or not.
Change the passwords and don't allow your browser to save them for auto-login.
If all else fails you can install a keylogger on your computer. It will record every keystroke made on the computer. You can turn it off while you are on the computer so it just monitors your children's computer time. It records the websites, emails they have written, and instant message conversations. You can find a free version on download.com . But since you are expecting to have your own privacy you should be willing to offer your children some privacy of their own as well. Hope this helps.