It is a hard concept for teens and tweens using the Internet to understand. Heck, it is even hard for some adults to understand, but the reality is there is no delete key on the Internet. Something you put out on the Web, even for a brief second, can be screenshotted, cached, archived and more.
It is best to implement the 3 second rule. If it takes longer than 3 seconds for you to determine if it is appropriate for that social media site, then it is best to not post it at all! View this tip sheet for more information on how to make sure your child's actions on the Internet won't come back to haunt them.