Why is cursing bad? They're just words. They're just sounds you make with your mouth...who cares!
Most cursing refers to sexual acts, or toilet functions. However, these references do so in a dirty connotation. The "F" word represents a different sentiment than "making love." One is disrespectful, uncaring and violating, while the other is natural and beautiful.
Now, very young kid's shouldn't be thinking about sex, but when they do, the association needs to be healthy and positive. And the same thing goes for toilet functions. The words that we choose to use for these things represent how we feel about them in general.
There is also a stigma. People who curse are seen as more unrefined compared to people who don't. On the other hand, cursing can also symbolize you as being "tough."
Cursing makes it look like you have a weak vocabulary. Many times it's easier to use a curse word instead of searching for the most descriptive and appropriate adjective. And cursing can make it look like you have weak self-control.
"Why is it worse for kids to curse than adults?"
It shows that your kids are interpreting natural functions in a negative way. It shows that you are allowing your kids to know about, think about, and do things that are reserved for adults. If your kids curse it can make you look like a bad parent to others.
Summary
Kid's use of language is learned in the home. There are different levels of cursing. We all know the most shocking one...but what about "Hell" and "Damn." Are those a curse words? Yes, but they are considered low-level curse words. "Shit" is considered a medium level curse word.