Learn English – the difference between you betcha and I betcha

I think sometimes they can be used interchangeably, at least informally.

The verb goof has several meanings, none of which are particularly flattering. It can mean:

• to make a blunder (as in, "Oops! I goofed...", also sometimes called goofing up),
• to joke around with in a teasing way (as in, "I'm just goofing with you...", also sometimes called goofing on, or goofing around with),
• or to waste time idly (as in, "We were just goofing at the mall", also sometimes called goofing around, or goofing off).

According to many dictionaries, the phrasal verb goofing off adds to a measure of irresponsibility to the simple act of fooling around – it implies loafing, particularly when one should have been doing something else. However, I'm not convinced most native speakers give much thought to that subtle distinction very often.

According to strict dictionary defintions, then, a group of teenagers hanging around on their skateboards on a Friday afternoon might be said to be goofing or goofing around. However, on a Thursday evening, that same group of teenagers might be said to be goofing off (assuming there's unfinished homework due on Friday).

Here's an Ngram you might find interesting:

As you can see, there's not much distance between the green line and the gold line, showing that, more often than not, the word goofing is used with another word, as in the expressions goofing off or goofing around.

It's a contracted, or shortened, form of "them" that's used in conversation, much in the same way that "gonna" is often used to mean "going to". In your quote, there should be a space, so: "Let's kick 'em." The apostrophe is to show a contraction.

So yes, it's correct to use, and often used in conversation. However, if you are writing it, it should only be used to directly quote someone who has said it.