I'm going to make a joke about a coworker's kid
Why? Is there a reason behind that?
Haha, yeah, there is. I saw a picture of the kid on my coworker's phone, and I can't handle how funny he looks
It's wrong to judge people, especially children, on their looks. Your coworker clearly loves his son, and making inappropriate jokes like this will only cause hurt feelings to your coworker, and problems for both of you at work.
It's not a big deal, I'll just poke fun at him and he'll recover from it. I have to have some fun in work.
The workplace is not an arena in which you should be trying to have fun, and certainly not at other people's expense. You need to be professional and considerate of others' feelings. If you proceed with making fun of this person's child you will only make your own job more difficult because others will lose respect for you.