Windows 8 parental controls are helpful, as are Windows 8 user account control features. Learn how to use them both here.
If you have multiple user accounts on your computer, it's easy to switch between users without signing out or closing your current apps. Switching users will lock the current user, so you won't need to worry about someone else accessing your account.
You can switch back by selecting the desired user from the drop-down menu. You'll need to re-enter your password to unlock the account.