Since you're not the main authority figure for your grandchildren, you can afford to have a little fun. You can enjoy the adrenaline rush of breaking a few low-level rules with them, like staying up past bedtime for a special movie night, or teaching them something their parents might find crass, like spitting watermelon seeds. While it's fun to act up with your grandchildren, make sure you don't cross the line into being disrespectful of your children and their parenting choices.